How small is my sister for her age?

She might be a little short, but I wouldn't worry about the weight. My sister weighed about that when she was 10. She's 17 now and hasn't even hit 100 lbs yet! She's always been tiny but the doctor said she doesn't really have any problems. Your sis might need to go to the doctor just to see if anything's wrong. There probably isn't but just to be safe...