My attitude is you train and try, but you have to have a good instructor.
I know you said your sister's not in a wheelchair, but that wouldn't matter:
http://www.youtube.com/watch?v=wrOanivzFmE
The instructor will make all the difference. Someone who is patient would be really good (There's a guy named Valentin at the Butokuden who teaches a children's class for the Orange Coast Aiki-kai who I feel would be perfect if you're in the Southern California area – the guy has infinite patience, and an incredible warmth about him).
Aikido is exhausting, and does have a lot of exercises. It's extremely demanding, especially with taking Ukemi. I believe that, if she falls easily, learning to take Ukemi will be very good for her for preventing injury.
I want to say this before I close: Let her try if it's what she wants. Life is short, brutal, and painful; and being denied those few joys in life on the grounds that someone else feels you can't do them is insufferable. We learn by trying, failing, and trying again. Eventually, maybe we get it right. Give her the opportunity and watch her shine. I say this because as a kid I was tall for my age, and told that while everyone else got to do tumbling in PE that I should just sit to the side because I'd never be able to. Now in training, I roll, cartwheel, and flip as readily as the other students. It just took practice.