Question:
Can a divorced spouse attend his re-married wife’s seder with her new husband in their home along with the divorced couple’s daughter, son-in-law and infant?
Answer:
Please see the following post, where a similar question was addressed, and the answer would be the same.
https://dinonline.org/2018/03/19/eating-a-suda-together-with-the-kids/
Best Wishes