I don’t have direct experience but have friends who have dealt with this. Unless everyone is the family is treated consistently the child will just keep getting lice again. If the family isn’t willing to take all the measures (and it requires hours per day and lots of vigilance) then I think the best thing you can do is protect yourself and your house and try to help the kid when she’s in your care. As far as how to protect yourself, since the child is in and out of your home you might want to contact a professional for advice. It can help to keep her hair in a tight ponytail and covered with a bandana. No head-to-head hugs and don’t let her sleep on your couch, in your bed, touch her head to the floor, etc. 🙁
I have a friend in NYC who had GREAT luck with a professional nitpicker. The kids ate Oreos and watched movies. First time through got all the lice and nits. She checked the rest of the family for a small additional fee and at the next checkup they were lice-free and have been since then. It wasn’t cheap but it was a far better option for them than spending hours per day sorting through every strand of hair on every person’s head.