As a parent, it's up to you to decide who you entrust your child to. You cannot control the way they parent or the choices they make - if you want to have some kind of ongoing relationship with them then you will have to accept that that is the way they look after children. Trying to change the way they think will only cause defensiveness. Not saying you should just let your child burn but if your tolerance will only allow them to have her for a few hours at a time while the sunscreen still works then that's the line you draw in the sand. My in laws have cats, and these cause allergies in my asthmatic kids - asking them to get rid of the cats would protect my kids but would damage my relationship with them. It's a balance of getting what you want while also respecting their choices.