So...I'm thinking of coming out to my sister before my college graduation...what should I say or do?

Trust me, they all know...no big deal. Invite them all, it is unnecessary to "come out" to those who already know. If you make a deal of it, it will seem as if you have something to make a deal of...and you don't. Just leave things as they are, nobody is asking, nobody needs to hear it, for that might be construed as insulting their intelligence. NOW, if you were straight and they thought you butch, that would be a different story.