You must priortise YOUR OWN CHILDREN. Must. If that means splitting from him, so be it. They cannot and must not be forced to share a home with an aggressive, confused, traumatised boy. Your job is to look after your own kids. You already know the answer.
Your time, your choices, your emotional stability, your kids, your mothering - these are not resources that other people get to deploy as they wish. It is NOT HIS CHOICE to foist this onto you.
It is not your right to force your kids to put up with this either. You are their mother first, and his fiancee second.
Separate, you can focus on your children, he will soon get very sick and tired of parenting the boy. At this point, he is probably (almost certainly) planning for you to have tolerate and deal with the boy most of the time and will not be prepared to do it himself. I'd be surprised but pleased if I turned out to be wrong.
Obviously, it's shit for the nephew and obviously everyone feels bad for him. But there are a million others like him and we can only do what we can do. You can't parent him, so don't try to.