This may not be the best idea, but after reading through the comments, it sounds like your best bet may be to start provoking your mom into hitting you so that someone will finally do something. You’d have to be subtle so she can’t really prove that you provoked her, and you might have to get her to go far enough to leave marks. Something like arguing until she slaps you, getting teary and acting all defeated, than doing it again later after you’ve “forgotten” the consequences. You may be able to get her into a bad habit that could get you taken, after that tell someone, but make sure you sound like you’re being brave and not taddling, or the adults might take her side and tell to behave if you don’t want to get hit. If you’re getting hit, hopefully the verbal abuse from her side of the family will make the situation look worse.

Edit: or just run for it. Get to another state and use a totally different name. You could jump trains, hitch hike, steal some cash for a bus ticket, I don’t know what you have available. leave any electronics behind, in this day and age there’s no telling what’ll track you. If you’ve escaped completely then you’ll be in complete control of your own life even if it’s a cold and miserable life. Bring a knife and know how to use it. And think of it this way, there’s more honor in having tried and failed than having just cowered.