Mental Health Support

I’m very sorry this happened to you.
Depending on the personalities and habits, raising kids can feel like being lost in a maze or a ship lost at sea, and one just tries to survive day to day.
Or partner / spouse may be interfering.
It sucks nd is very rude.

But the nephew especially needs you, or will later. To be a part of their lives, you need a small but regular ritual with them, to keep reminding them what is extended family. It doesn’t have to be in person. For now, you could consider a ritual of friendly funny postcards (the amusing uncle), or send a photo or memory of their uncle/mom/grandparents when young (roots), or encouraging notes (the caring uncle), or little mechanical puzzles and scientific oddities (the scientsy uncle). Not a lot of money, just repeat connections. They may need you LOTS some day, one never knows.

Phases happen. You can and will have a chance again. Just be there for them in a quiet consistent way so they know they can count on you to care, when they need. Non-parent to talk to.

Part of being a parent is seeing past the present and seeing the possibilities of the future. The same is true of being a good uncle.