YTA all day. Your brother is also the AH - he did betray you.

Still, that was then and this is now. Clearly your brother and Emily are more than a short term fling. You're going to have to find a way to either get along with your brother and Emily, or be prepared to go fully non contact.

Emily is a person who's reaching out to someone she feels close to, a member of her family, in her time of need. Yes, she can receive emotional support from your brother, but people are allowed to have a wider emotional support network as well. You're intentionally being cold and cruel to her because she didn't fuck you all those years ago, even though she never led you on, even though you never made a move. You need to grow up.