https://slate.com/human-interest/2018/03/parenting-advice-on-abusive-histories-mom-friends-and-grubby-toddler-fingers.html

It seems extremely clear to me (and I hope to you) what your responsibilities toward your son are: Kids should never be used to prop up the emotional or psychological health of an adult, and you are on this earth to protect him from harm. It’s less clear to me what your responsibilities toward your brother are as there are so many variables at play: Is he being cared for adequately in their home? Do they have a long-term plan for him other than that eventually they’ll die and you’ll sort it out? Do they have guardianship of him? I do not usually recommend dropping a dime to Adult Protective Services except in very dire circumstances, but if he’s violent toward Reed, he may also be violent toward his parents and probably himself, and I would be very surprised if he’s receiving the care he needs and deserves. Just something to start carefully probing. One final note: In my experience, people who have suffered abuse as children often react very strongly when their own first child is placed in their arms. Those of us lucky enough to have had good—or good-enough—parents are often suddenly struck by how much our parents love(d) us, and how many mistakes come along with the job, and develop a whole new appreciation for the work they did. People who’ve been abused as children often go in the opposite direction, as they realize how tiny and vulnerable kids are, and memories they’ve pushed aside or moved on from can resurface in a powerful, traumatic, and angry way. Just be ready for that if it comes. Prepare your partner if you have one, talk to your therapist, be kind to yourself. I’ll be thinking about you; please stay in touch