I must say I think that is so horrible of him to accept the invite. Maybe he will change his mind about attending when he realises it gives you a chance to talk with his girlfriend.  or if he fears it will force you to talk about him to your family.

Toward the light,

If its your nephew and your family and you want to go I would consider going.  As an inlaw he should feel like the odd man out. Let him bring a woman,  man, purple alien.   If it's too traumatizing then don't go..no need to look at him.  Sounds like a real loser for accepting the invite..and your nephew sounds odd inviting an ex.