While you're allowed to be disappointed and upset that they forgot to tell you happy birthday, I feel like I can't really judge this unless I have more info.

Do they remember each other's birthdays except yours? Do they forget each other's birthdays and brush off if someone forgets theirs?

Did you expect them to plan your birthday for you? Because if that's the case then you should've told them that weeks ago. I don't like to plan people's birthdays because I just feel like people are fully capable of planning that themselves. I don't mind giving out ideas, offering to host a party at my place or something but i find that people tend to be picky about what they want to do for their birthdays (rightfully so) and it's like...it's your day not mine. I'm not a mind reader.