Suzanne "Crazy Eyes"

Does anyone know why Suzanne lives with her sister instead of her parents? Just seems kind of odd to have someone with special needs living with their sibling rather than their parents (if they're alive, available etc.). It seems like she had a good relationship with them. I know she felt a little pushed by her mom but they were still kind to her and treated her with respect and love. I just found it strange that when Grace was leaving for the weekend she had to convince Suzanne that everything would be okay because she had her schedule and games and everything set up. It got me wondering why she couldn't stay with her parents while Grace was away if they were all worried it would be a problem. Suzanne seemed nervous about it and Grace was obviously concerned enough to make sure things were set up a certain way to be easier for Suzanne. So when I was wondering why she couldn't stay with her parents over the weekend it made me wonder why she wasn't living with them in the first place instead of Grace.