Lice infestation! Yucky.

Okay so I know this is probably kind of yucky to talk about. . .but I'm going to do so anyway. So my little sister in law who is about 9 years old got lice from school last year. My mil was cleaning her, then stopped. So all my mil's daughters got lice. Including her 2 year old granddaughter. . it's been almost a year literally and they haven't cleaned themselves. I've got lice twice because of them, and I cleaned myself. Now it's rare when I go to their house, or if I do, I don't sit on their couches nor put my head near theirs. My husband and I have told them to get cleaned over and over again. For them to use mayo, or the medicines they sell at the store but they never do. I recently got an idea from a friend. . . she says that I should call my sis in laws school and make a complaint. That way my mil is forced to actually do something about it. Should I? Or is that too messed up? Help!