Would you be mad if a family member thought you were the free on call sitter?

Okay, my sister and I are both stay home moms. I have watched her kids hundreds of hours without asking for anything. She is not allowed to babysit for me because to be honest, she is lazy and would not give my son the level of care he is accustomed to. I don't mind watching the kids when its something important, like today - she had a dental appointment. But now I am p***** off because I just called to find out she swung by my mom's work after her appointment and took her to lunch. I am ok with helping and love my nieces but I don't like feeling like i'm being used or taken advantage of. Do I have a right to be angry? Other times she has done similar things and left me with the kids 8-9 hours at a time.
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She obviously doesn't realize that it's an issue for you. She's probably just gotten so used to you saying yes everytime she asks you that she thinks nothing of it. You have to figure out some way to make her understand that its not okay for her to rely on you so much. You definitely have the right to be angry and I would if I was you. A good way of handling it might be to say no one of these times, maybe if that's hard for you then you could make up some reason why you're busy. If you say no a couple of times maybe she'll start realizing that she can't rely on you all of the time. Just because you are her sister and these children's aunt does not mean you should be obligated to take care of them all of the time. You are being taken advantage of and if you let this continue it could really break down the relationship that you have with your sister because you will build up resentment towards her for this.
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