My friend’s brother was taken to the hospital recently, in a coma. He never regained consciousness but lived for 2 weeks. The hospital had no problem talking to her and her sister (both parents dead for a long time), the doctors met with them and let them make all decisions, including when to stop medical care. The women hadn’t seen their brother in 5+years (my friend just happened to run into him 2 days before this and had learned where he lived). They didn’t have anything listing them as POA, next of kin, living at the same address. Nothing.
They were wary of claiming any legal guardianship of him because they needed him to qualify for medicaid. Hospital didn’t make them sign anything and I think were glad just to find someone who cared.