TWINS... how can I make my twin understand.....

I just told my twin sister tonight that I was in process.... she doesn't seem to get it, and I guess I don't expect her to. She has always been the thinner one. ALWAYs! She has always been 50lbs or so smaller than me, and she has always managed to let me know. Part of me thinks that she doesn't want me to do this because then I could possibly be the thinner one and she be the heavier of the two. Maybe that's true. But then she said, "I think if you do this, you are going to die." And I guess that scared me because she made it sound like I was being selfish, that I would risk my life for this. Is that wrong, am I selfish? She said I wasn't keeping my new nieces interests in the right place, she said I wasn't being fair to my mom. But then she wrapped it all up with, "but do whatever is going to make you happy, I just think you should re-think this, its a big decision..." Out of anyone in this world, I take my sisters opinion the highest, but there is a difference in our lives, I have PCOS, and she doesn't. She just had a child, and I have tried for years and haven't been able to. Is there someone out there that can help me, or give me a shoulder to lean on? Any advice?