Selfish SIL

I have the most easy-going, wonderful family…. except one sister in law that throws a wrench into everything. Our family loves her of course but out of sympathy for my husband’s brother that is married to her, we all end up accommodating to her just to keep the peace. Most of the examples are petty things but over time it can wear on you. For example, we all bring an item to contribute to Sunday for dinner but she seems to either be busy, or had a bad night’s sleep, or blah blah that she usually does not contribute to Sunday dinner. She just comes and eats. Not a big deal but kinda annoying. I get irritated at everyone bending over backward for her, but really if it does not concern me it shouldn’t bother me. It makes feel frustrated that I get so irritated by her. I know that my thought needs to be about not changing her but I need help building a model to get me there. Right now I just find myself annoyed with her composing and whining all the time and I can’t get ahold of my thoughts. Help