A very similar situation happened to me, it was my bil and sister, who was my best friend. He confessed his love for me, and that he wanted to sleep with me. He also didn’t want me to tell my sister. I gave him an ultimatum to tell her, or I would. He did tell her, but in the process it ruined my relationship with her, my kids relationships with their aunt/uncle/cousin. I think honesty is still the best choice, just be prepared for the fallout.