You set a boundary, things are quiet and if he responds by again invading your boundary than don't engage.

That's what I had to do with my older sister, I learned that it was best for me to stay away from her entire drama when my parents were dying. Same thing with my parent's funeral, it was not for my parents at all instead it was just another one of my sister's DRAMAS. I had a good therapist to help me navigate all that toxic and then I had to hire a lawyer who also noticed how incredibly toxic my sister is. It's gotten to the point where everyone sees it and it's embarrassing that I have such toxic older sister. Even her own family doesn't like being around her.

You have every right to defend your boundaries. I have been slowly getting to the point where "let her have all her trophies, she needs them way more than I ever did". I have learned how she has a bad reputation, no one wants to work with her in her field. People do whisper to each other, "stay away from that one".

If someone is vengeful, then get away from them. Let them think they won even. It's not worth it to have any contact. For myself, the hardest thing was/ is the person they are hurting, be it a child, a pet, or a frail aging parent that can't handle the toxic. That's what was the hardest part for me, how her behavior affected my fragile aging parents. I understand that is my challenge and not yours, however, I had to figure out how to navigate all the toxic in a way that preserved my own mental health. It's not always easy, and the outcome may not be ideal. There are things we cannot change even when we want to.

If he responds, don't answer, you can share here and perhaps we can help you respond in a way that can defuse if the response is hurtful. I think you pretty much know, it's just nice to have support. So far your nephew is now at a place he can get treatment. That's a plus that you helped happen.