In this book, Moose's mom expects a lot from him. Although he is the younger child, he is expected to do a lot for his sister because she is mentally retarded. Now since Moose's mom cannot take care of her, she tells Moose to. On page 85, it says, " 'Gram doesn't live down the street anymore, honey.' My mom sighs. 'We can't do this without you. Being around kids is good for Natalie. Mrs. Kelly says so. And if she's to get accepted in the Esther P. Marinoff...' " I think this shows that Moose's mom trusts him. She is telling him that he should take care of Natalie, so she thinks that he can do this task. She thinks that Moose can take care of Natalie, even though she is mentally challenged. I also think that the quote shows that the mom believes that Moose is responsible. She is leaving Natalie in the care of Moose so she has to believe that he can take care of her and will not get her in any sort of trouble.
http://thenovelworld.com/2009/03/22/al-capone-does-my-shirts-review/