I must disagree about Religion being one of the "most dangerous things in the world". I think in terms of this, it is how people take things that is the problem.
I am sorry Jeh to hear about what your sister said.
I did have a similar situation years ago. I have a twin sister, we have always been so close, as she is my best friend. My whole familly struggled with this,even having known for a long time.
Me and my siblings and Mum are all Christian, and they did view my transition from this point, even to the point where they worried about me and my faith (and I admit, transitioning does still effect me and my faith, making it shaky at times, but Ive just got to press on and try not to be weak I guess :p)
My Mum said she felt she was losing my, and my twin did. They felt that this wasnt what God had made me for, I shouldnt change what He created.
My twin went to uni (I took a gap year) and once she left, and I started T, she said she doesnt think she will ever be able to see me again with T effecting me etc.
That broke my heart.
But I still kept on, I felt I was losing my family, the only people I had at the time. But not transitioning was no option for me. I tried to keep them in my life and I can say, that hopefully my life can be viewed as a story with a positive outcome. Hope paid off.
Though at the beginning, my friends left me and I was made to move out and my Church didnt accept me, now, me and my family are very close again my sister and Mum have come round to it all, they accept me (and so has a new, better, less judgmental Church)
Though I know my family fully felt and meant what they said at the time, they just needed to see how much better I could and would do after beginning transition, and that you were NOT doing anything wrong. And most of all, that you are the same person, only happier, more productive and still able to be loved by them.
Easier said than done, but you need to stay strong, show them you can get through this. I think things would have been much better with their acceptance, but if you dont have your families, then, respect how they are feelign but do this for yourself and try to keep in contact with them, so they can be brought through it all along the way. The changes are less drastic that way.
I hope this has been of atleast some help.