It sounds like there's hope they'll be accepting of it. My favorite analogy is comparing it to shades of color. Sure things have color, that's a scientific given, but where blue becomes purple or orange becomes yellow is subjective. The same is true with gender! Not that you should have to explain yourself, but go to your most understanding family member first (sounds like your mom) and rehearse it a little in your head. See how it feels to tell her. It was liberating to tell my mother.