I have the same issue with my sister. She wants nothing to do with me unless she can control me or get something from me, and she constantly passes judgment on me for not living my life to her standards.

I honestly only invited her husband and son for holidays once. She never knew. I suggest you do the same, so your nephew doesn't believe you hold something against him, too. I'm sure your SIL already knows. I think it would give them both a reprieve. Suggest they tell him they're going to see someone he dislikes, to discourage him from trying to join.

No point in surrounding yourself with toxic people, but also no reason to isolate others for something that wasn't their fault.