And don't think the resentment of forcing badly behaved kids into the family home ever goes away. No matter the reasons, no matter the sympathy for the nephew, or any other child, it's a bad idea.
I used to work alongside a man called John, a good guy, kind and caring. When his kids were around 10 or 11 he and his wife decided they wanted to foster kids to offer children who needed help the sort of stability his own had.
His own kids appeared to tolerate the continual disruption quite well, or at least any complaints fell on deaf ears.
Years later, after he had stopped fostering and his kids were young adults the conversation came up - and both of them told him in no uncertain terms how much they loathed every single second of being forced to share their home, their parents, their life with badly behaved, traumatised children. They resented him horribly for it, and though they loved their parents felt strongly they should never have been put in this position.
His choice (along with his wife's agreement) permanently damaged their relationship and John and his wife felt forevermore they had to somehow make it up to their kids.
He told me all this with deep regret at a work party over a few drinks. He would have done anything to change his choices back then and prioritise his own kids, but the ship had sailed.
You can't make it up to your kids once you remove their safety, their childhood home, their stability, once you unfairly (as they will always think of it) divide your attention with other children.
You can't fix the world, you can't foster or adopt all the damaged, unhappy children. You CAN however parent your own well, safely and provide stability for them.
So do that.