Having been involved in a conversation about this sort of family dynamic with police, I wouldn’t recommend it. Unless OP is in a gigantic city, he is not going to receive any help from the police that actually protects him. What will happen is the police will listen to his story, then go talk to the other parties involved to get their story.
If an officer believes him right off the bat, they can/may hold off on talking to the rest of the family and tipping them off that OP is making these claims, but even in that case OP still may be stuck in the house. They might be able to connect him to a shelter. Might. The problem is that he is male. Male domestic violence shelters are almost certainly going to be a shit-show at best, and 95% chance there isn’t a shelter for males of domestic violence within any distance that the cops would help him get to.
The best bet for OP that involves cops is to follow Chonk’s advice: https://links.hackliberty.org/post/3089471/5115602
Use the cops for what they are actually good at: protecting you in the exact moment they are there. Anything else is going to be a time-consuming investigation, unless the brother/father literally admit to planning to kill OP.
The obligation to treat patients who are in need in an emergency setting IS a legal obligation in the US. If a patient is refused treatment at an emergency room, both the doctor and hospital can get gigantic fines. I don’t remember the max off hand, but it’s somewhere along the lines of $50,000 and $1.5 million, respectively. The law in question is EMTALA, or the emergency medical treatment and active labor act. A patient must receive stabilizing treatment, or be stabilized to the best of the hospital’s ability and transferred to appropriate care.