Kaori, Sounds like both of you care about each other. How often is it that a family member is or has dated a trans person, if the person was in the initial stages of transition, it would be understandable your cousin would be frustrated. Perhaps it may take nurturing her that transition isn't the same for everyone. At least she is trans knowledgeable from her experience, so open to this & she will always have the same soul that is you. With a bit of time she may adjust, I don't see any negatives here. She could be your allie when you come out to family. We don't hurt anyone. Hopefully family love will win on your coming out to family. Don't give up.
Good luck.
Jamei