In your post you said you were out to your brothers, so I'm assuming you are the only afab child. It's likely the pressure from your mother is because she wants the daughter narrative from you, to connect via what she perceives as a shared femininity. Of course it isn't fair or appropriate for her to push an agenda on any of her children, she probably wants to connect with you the way she knows how. My suggestion would to be to reach out to her. Find other ways of connecting and relating to her that doesn't involve things that make you so deeply uncomfortable. It's hard to look at the other perspective because it seems so unfair, but figuring out Why she's badgering you about it can be the key to redirecting the energy to something more positive for the both of you. It will most likely hurt her to learn she does not have the daughter she very obviously wants so be cognizant of her expectations when you Do decide to come out. I would very much suggest again, to find common ground with her outside of femininity so once she does learn she has another son, she still has something to hold on to, replacing the daughter she thought she had. That's my two cents and if I'm wrong about family dynamics here I apologize. Hope it helps a little none the less.