getting insurance for a child that isn't mine, need advice.

Is it possible for my fiance to get insurance for his little sister (11) or do we have to pay out of pocket for everything? We don't have custody of her (yet) her aunt and uncle do but they refuse to take her to any doctors appointments. She needs to go to the dentist for cleanings and x-rays and they won't take her. she hasn't been to the dentist in over a year almost two years at this point. She has asthma (because she was a premee) and her throat keeps closing up and making it hard to breath, I think it's just cause of the cold but she still needs to be seen by a doctor for it and lastly she now has a scalp infection because the salon they take her to can do shit right when it comes to her hair (she's half black) and her aunt and uncle are choosing to ignore the scalp infection rather than deal with it. Sorry for the rant but I felt like you guys might need to know why we need to get insurance on her. Are we even able to get insurance for her or are we going to need to pay out of pocket? I mean if we need to pay out of pocket that's just what we're going to do and obviously it will help us build a case for us to get custody of her but it'll still be a while until that will be accomplished since we have to build a whole new room on our house and of course buy all the things she needs, but at this point the only thing we aren't doing is housing her during the school week. SMH. also if you guys have any more advice on building a case or little extras that will help us or even organizations that will help us if there are any that would be fantastic because this is so hard to do and we are both just barley adults (18 and 19) sorry for the whiplash of a story but I didn't want to drag it on for too long