I did the same thing kind of with my mom... I eased her into it, first by coming out to her (but not dressed feminine or using feminine voice). Then slowly over time we have been talking, she is getting to know me and my situation more and more. She's reading books and watching documentaries on trans topics... I slowly introduced her to my new name, and visibly seeing me en femme. She's really coming around. She will never truly be able to sympathize with me, of course, but at least she can gain an understanding of why I felt and feel the need to transition.
A lot of people need to be eased into the idea... Transition is extreme to most people, and they're going to be shocked. In my opinion, the slow approach has been best with everyone I know. I feed people little parts of my life, and let them process and assemble them on their own, in their own heads. This way, they've come to their conclusions on their own. I think this really helps gain acceptance, giving people time to deal with it in their own way.
I did something similar on Facebook... I just canceled my old account, and created a new one. Then as word spread about me, little by little, the people who wanted to maintain contact with me started to add me as a friend again. So far, I've gotten almost everyone back, but it took over a year.
Having the rest of your family on your side will help with your sister, because your mom and brother will advocate for you when you're not around.
I think you're doing the right thing. You've just got to plant that seed in people's minds, nurture it, and let it grow... instead of digging up their yard.
