*Spoiler* Question about the film

I just finished watching this and I had only one question after it was over... Why was Jimmy making his 13 year old niece Carolyn administer the medicine to keep Mick's mom in a catatonic state and then possibly kill her later?? Why didn't he have his sister doing it? Even if she didn't know what she was administering to Mick's mother and was thinking it was medicine to make her well, she'd find out later after the old lady died and realize her role in it and probably be just as screwed up as him for the rest of her life. I'd think having his sister do it would make more sense because I imagine she's as messed up from their childhood as he is and he can't really screw her up anymore.