I'm mad at my sister when she didn't give my kids the dinners I had left for them while she was babysitting
Is that the only thing that made you mad?
She was not exactly nice to them during the time she was their babysitter
Family is supposed to be there for each other, so it is okay to be mad when they do things that make things harder for you. It would be better to talk to her so she understands how you feel and actually give you a hand.
I want to talk to her.
Right. Just remind her gently of the times you've helped her out. And let her know exactly what you expect of her.