Handicapped Sister?? Advice?

My little sister is handicapped, she has cerebral palsy and characteristics of downs syndrome. She just had the final part of a two part surgery for leg correction in which they removed the plates and screws leaving her with 22 screw holes and 2 slots where blades were inserted. Everything went fine but now she has to stay bed ridden for a few days until she can walk again, and mind you her walking is not good, never has been. But anyways, as she's gotten older she has become increasingly aware of her condition, (she has a full vocabulary and everything, she speaks well and understands) and she's started to say things like "why am I trapped in this body" "Why was I born handicapped" "I'd rather I'd died instead of been born this way." As her older sister, this is hard. I don't know what to say to her anymore, shes almost 15, she's smaller than normal, her legs are a little atrophied, I mean it's obvious that shes different and it seems like shes become very aware of it this year, probably because she just started middle school and you know how kids are. I just want some advice, how do you comfort someone who's never going to get better? How do you explain the 'why' part. Sometimes I find myself asking God the same questions she asks me. I just wish I had an answer. Any advice. So far I have no idea what to say to her. She's always frustrated because she can't do the things we can do. You'd think that after all these years we'd know what to say but even my mom has a hard time giving a good reason. Any advice would be helpful, thanks in advance.