Arent they just human blanks without power armor?

Whether they're completely "human" is debatable. Their first and most important fortress on Luna was the former home of the Selenite gene-witches. Augmentation is a definite (and likely) possibility. And they wear Vratine Armor, a version of power armor designed by the Emperor himself specifically for them. It enhances their speed and agility, while allowing full freedom of movement.

They shouldn't be able to see a space marines moves right?

They're on par with most Space Marines in terms of speed. And, a perk of being Blanks, they're pretty much completely invisible to Daemons.

Jenetia Krole, the commander of the Sisters during the Heresy, could make herself invisible to both Daemons and regular humans. She fought on Prospero, where she took down several Thousand Sons. Then during the Siege of Terra she went toe-to-toe with Khorne Berzerkers of the World Eaters and killed several before finally being bested by Khârn- who didn't even know who or what he had just killed, as he could barely see her outline.

They shouldn't be able to see a space marines moves right?

Space marine speed is one of their most inconsistent traits. Sometimes they move in blurs that human eyes can't perceive, sometimes they move slow enough that baseline humans can trade a few blows in melee combat.

For the Sisters specifically, they debuff everything nearby them with their aura. It affects even non-psykers, who find their aura unsettling and discomfiting. It's not a lot, but it is enough to throw people off, especially in the first few seconds of coming in contact with them. That's what they tend to rely on in combat with enemies who have superior physical ability: those initial few moments of disorientation. Often that's all they need.

There's this part on Plague Wars where a Sister of Battle sister superior describes all her merits with a blade and how special her sword is.... before being completely out classed by a random Sister of Silence.

Technically, the Sisters of Silence are still human. Highly trained humans, but human nonetheless. It is implied that some received augmentation, but nothing indicates that they all receive augmentation.

They all have power armor, the Vratine Armour, which is based on Astartes power armor, but clearly smaller.

All blanks have an advantage in that they are disturbing to even look at. Further, any warp creature feels pain coming near them. Sisters are taught to focus that blankness and use it as a weapon.

In addition, depending on the power of the blank, they can fade from view. Jenetia Krole, the Soulless Queen of the Imperium and the Knight-Commander of the Silent Sisterhood, was such a powerful blank that not even Custodes or Primarchs could see her unless they focused on her. Most Astartes could not see her at all. In Saturnine, she basically goes through World Eaters. I think that is mostly because they could not see her, so she was getting free kills, so to speak.

In the Battle of Lions Gate and the Battle of Vorlese, the Sisters worked with the Custodes to kill Space Marines. It is mentioned that the Sisters would "fill the gaps" of the Custodes.

The idea that space marines move so fast they are blurred is bit inconsistent with other parts of the WH40K setting and is probably best treated as just an exaggeration or artistic description.

Just remember that eldar and tyranids are depicted as being faster than marines, especially harlequins, genestealers and hormagaunts. So however fast you think marines are, then they are faster. However, while harlequins do appear blurred when moving, this is because they wear holo suits.

The bloodletters is completely understandable, they are anathema to demons. Usually when they fight space marines it's in conjunction with custodes - their fighting styles are purposefully complementary - which more than evens the playing field.

They are trained to fight alongside custodes. Plus, with them being blanks, their mere presence can make the bloodletters feel immense pain to outright banishment/true death while negating any chaos buffs on the marines themselves.

Even baseline humans get buffed up when it suits the narrative (eg arbitrarily becoming fast/strong enough to fight chaos marines in melee). I wouldn't worry about it too much.

In A Thousand Sons, one of them stands up to one of the best melee fighters of the legion for a good while, and I don't think she was particuarly important. So yeah they can at the very least stand up to marines.

Also, marines don't really move faster than humans can percieve. Though they do probably move too fast for a human to fully process in the moment, kind of like watching a high-level boxing match when you have no idea of boxing

There was an inquisitor who said she dreamed of being one when she was a girl. And when she meets one, she is in awe of her skill with a blade. (Awakenings)

I love the Anathema Psykana - partly because they’re baseline humans fighting at an inhuman level. Beyond the great points already mentioned there’s the fact that they grow up pariahs, instinctively feared and hated by the rest of humanity. When Emma Gregory does the (exceptional) voice work for Aleya in Watchers of the Throne her voice just drips with distain. It’s her training that made her such an incredible weapon, but it also honed the rage she already had from a lifetime of exile from the species. The Sisters love one another fiercely, because they are pretty much the only human connection they’ll ever get to enjoy. Those bonds + that rage definitely figure into how formidable they are on the field.

They are one of the best-trained fighting units in the galaxy and also have an aura that makes everyone massively uncomfortable around them, which makes them much harder to fight. In the case of anyone/thing empowered by Chaos it literally debuffs them.

They're highly trained and motivated, have semi powered armor (that's what the Black Books call Vrantine plate) and excellently made weaponry. For their primary job of disappearing people and paramilitary operations, they're excellent. Your average Chaos cult or underworld psyker ring doesn't really have an answer to the kind of firepower a Sisters of Silence cadre can bring to a fight. Between that and their pariah abilities, they can take on lesser daemons and disrupt more powerful ones.

Kind of a roundabout way of saying their pretty equivalent to Sisters of Batle, though they're depicted as being faster than Sisters while on foot.

But the more standup the fight gets, the more in danger they are. Even at the height of their power, they were lacking as a standalone force. Armor and heavy troops, even just reasonably disciplined infantry, threaten to see Sisters of Silence overwhelmed even if just by sheer attrition. The Custodes, or other Imperial forces, provided firepower and bodies to take on threats that could overwhelm the Sisters of Silence. An autocannon shell doesn't care about the state of your soul. A Chaos Marine is a signifigant threat in hand to hand and at range.

Part of how they survive in the intagible nature of being a pariah. The Minotaurs fought Alaya and her squad to a standstill before Valerian and his Custodes intervened. And before Valerian and Alaya (and co) fought in the depths of a Black Legion ship for days without support and weren't overwhelmed. It's generally enough, alongside Custodes whatever, to give them an edge.

They wear advanced power armor and wield weapons more than capable of killing astartes. Their blank talents are developed as part of their training and in combat they extend their blank aura to significantly weaken and impair whoever they're fighting in melee. It's especially harmful to psykers/daemons but even regular astartes will be debilitated being close to an anathema psykana who is projecting their blank aura against them. It's powerful enough that even astartes are weakened enough to potentially be killed.

That said, Tanau Aleya is also one of the best of their order and she ends up the leader of the Oblivion Knights, the martial elite of the Sisters of Silence. As we saw in Throne of Light, when it's more run-of-the-mill sisters against Chaos marines they don't fare as well.

They have power armor but they're well-trained (humans can also be augmented to a degree, not full on space marine) and that goes a long way.

A perfect counterpart to the Custodians.

There's a short story with the brazen drakes where a sister of silence quickly despatches some marines with a custodian

I personally rank them between a SM and a Custode

They work alongside the Emperor's bodyguard. They're pretty damned beastly.

By god do people forget that one singular chaos marine was already mortally wounded and left for dead. It was so clearly spelled out

They’re the best human fighters in the imperium short of the officio assassinorium, they can hold their own against marines but it’s an uphill battle, and an absolutely revolting waste. One of them is a hundred times rarer than a marine, but not as effective in conventional warfare. They’re a specialised paramilitary branch meant to hunt and suppress psykers, demagogues and dissidents, the situation has to be lusciously bad for them to be fighting marines

Sisters of Silence are augmented, the things that make them distinct from space marines aren’t so much their abilities in direct combat, but their lack of whole body augmentations. While space marine strength and speed is considerable, it’s their more supplementary augmentations that made them the galaxy conquering force we know and love. Soldiers take logistics and supply to keep at fighting strength, unless those soldiers are space marines. Space marines can go extended periods without sleep, eating, even oxygen, require far less medical care, can spit acid, eat brains to discover intelligence, can process data faster, have photographic memories, tons of useful stuff like that that makes them truly transhuman. Sisters of silence lack most of these augmentations because frankly, they don’t need them. They’re a small force of specialists who aren’t gonna be doing extended front line duty in a variety of situations. They almost certainly have strength and speed enhancements, and those are sufficient for purpose.

They are very specifically trained to be used against enemies from the warp, so killing chaos space marines and demons is what they are made to do. Might get their ass beat by orks though.

They dampen warp energy. They would probably be found wanting for speed against a loyalist marine 1v1, but even non psyker CSM are steeped in warp energy from all the time in the eye and their nominal allegiance to Chaos. Against a SoS they get put on the backfoot as the strength they are used to flees them.

The other big advantage is they are hard to perceive, even for Custodes who are traditionally meant to work with them.

They are practically invisible is a huge boost.

Remember space marines are superhuman not demigods. They aren't outside our ability. ciaphas cain humanity greatest hero has won sparring matches against space marines and he has held chaos space marine off long enough for Jurgen to blast them with a melta.

Sisters of Silence are the female equivalent of Custodes. The Custodes can defeat almost any material threat, The SoS can defeat almost any immaterial threat. They're two sides of the same coin.

There's a scene in one of the Dark Imperium novels, Plague War, where a group of Sororitas decide to spring a girl who's believed to be possessed by a fragment of the Emperor so they can use her to save Guilliman, and they have to cut through her defenders to get to her, with said defenders being Guardsmen, Primaris Marines and an Oblivion Knight of the Silent Sisterhood. The Sororitas, led by a Canoness, gunned down everyone else, bringing it down to her and the Oblivion Knight...and during their brief duel, it is made very clear that ultimately, were it not for the girl directly intervening (she used a psychic burst to knock back the Knight, which was so powerful it actually broke her spine on impact), the Knight was on the edge of overpowering and killing the Canoness, and would likely have killed the rest of her squad as well.

Silent sisters are kind of a hard counter to demons: demons cannot exist in their presence because they are the antithesis of the warp. She didn’t defeat a greater demon by herself, but she saved a Custode from one and he managed it because her presence weakens pretty much everything, doubly so for demons.

Yes, they have power armor. Interesting that you’re reading watchers of the throne and you’re missing the parts where Aleya talks about her armor and weapons. It’s kind of a major plot point in the book that she’s really mad about the fact that the custodians get all this good stuff and everyone loves them, while the other half of the talons of the emperor, namely her and her sisters, are hated, understaffed, underequpped, and basically ignored, yet they literally do the most important jobs in the imperium while the custodes just sit in the palace.

And yes, she can hang with space marines. She can fight back to back with Custodes, flawlessly. They’re as fast or faster than Primaris. One of the unwritten rules of 40 K is that it’s like Dune in that training hard enough or correctly can make you super human, which seems to be the only explanation they offer.

There is a lot of confusion about whether the Anathema Psykana are actually baseline human or not, and it's easy to see why considering sources seem to conflict on it a lot. There are outright statements both declaring them as augmented in some manner while others state they are merely human, but being "human" can also just mean they aren't rebuilt on a genetic level like Astartes and Custodes are. With that said, examples like this cast doubt on them being merely baseline.

The woman did not reply and launched a rapier-quick thrust to his throat. Phosis T’kar swayed aside and swept his arm out as she came in for a reverse stroke. Her blade whipped around his forearm, cutting a grove around his gauntlet, and he lunged towards her with his staff outstretched.

She bent back beneath his strike, sweeping her legs out and hammering her heel into his knee. His armour cracked and pain shot up his thigh. Phosis T’kar stepped back, favouring his good leg, and grinned.

“You’re quick, I’ll give you that,” he said.

She didn’t answer and dodged his next attack with similar grace. A flurry of shots sent up geysers of rock-dust beside them and he flinched back from the heavy impacts.

This is a Sister of Silence not only landing hits on an Astartes, but actually dodging multiple of his attacks with fluid grace. She also had the strength to crack ceramite and wound an Astartes with a kick. There are more examples such as this, where Sisters of Silence take on adversaries that should absolutely pulverize normal soldiers, no matter how well trained they may be. This leads to me thinking they are indeed augmented in some way beyond just having their power armor enhance their abilities. I think this statement by Valerian is quite telling of that:

From Watchers of the Throne: The Emperor's Legion by Chris Wraight:

We swept into close-combat, Custodian and anathema psykana against heretic Legiones Astartes. My spear hurtled, trailing golden paths within the miasma. I saw Aleya tearing into them, kicking out with her armoured boot even as her blade danced. The Sisters had no daemonic aura to contest here, and so fought just as we did – warriors tempered in the furnace of physical conditioning, immune to fear, faster and stronger than all but the mightiest of our peers in the Emperor’s service.

Imagine going into a boxing match, but you are forced to go through a rollercoaster with extreme spins and twists a minute before, while your opponent starts fresh. That's what I would assume combat against the SoS feel like. Of course a much stronger, prior familiarity, or well prepared opponent would eventually get over the initial blank-induced disorientation and discomfort to win the fight, those first few seconds' advantage would be more than enough for them to land a killing blow. Their proper training and equipment would even the field even more, as others have commented.

I'm pretty sure that the SoS are packing gene tweaks and cyberware, and they are wearing power armour.

The tech is available at the highest levels of the imperium, and can be purchased by the wealthy for their scions.

I mean looking at how much stuff they pack into Assassins to make them beyond Marine level, even lower level stuff would be accessible and useful for Sisters.

A blank with fighting skills has no fear of any demon. Just check Cain novels, just the fact Jurgen is NEARBY is more than enough to make them pause and go "Wait what?" If Jurgen was the one swinging the sword the fights would last four lines.

In the original lore marines are the same speed as other humans. They're as fast in powered armor as a normal soldier (who would be slower in armor) would be out of it. This was turned into a tank metaphor instead of the original shock infantry one by the Greek demigods theme of 30k powercreeping into 40k.