you feel guilty because you've been trained to... that's one of the ways crazy people keep you in their game.
keep in mind it's *their* game, you don't know the rules, you aren't allowed to figure it out, that's part of the rules... if you start to figure it out, the rules change. given that you can't know the rules, you will always, always come out on the losing side of the game.
the only way to win, the only way to survive with your sanity, is to not play at all.
I know this, first hand. I was raised by crazy people. so here are my suggestions:
1) stay away from crazy people. especially if they're related to you.
2) save your grandma. call senior services or the county welfare agent and ask for help for her. if she can't get up when she falls, she can't live alone. she's having significant health issues, she needs help. if you can't take her in, then get her help from someone else.
3) call the police on your brother. if he punched someone, that's assault. and in this case, it's elder abuse too. report it, ask the police to investigate, the DA to prosecute. if he can't behave like a civil person with the woman who's housing him and paying his way, if he feels free to punch her, clearly it's time for him to have another lesson. let the system do that. his behavior will continue to escalate, you've already seen that. the next time he's ticked off, it may go well beyond just one punch... getting the police involved may save someone real injuries or worse.
once you've done those 3 things, you'll know you've done what you can, and you can start to breathe again, let go of the guilt, and have your own life back.