I am in a similar situation, however I have been w my husband for 12 years. And we have custody of our step son who I have helped raise for all those years. My deliema is that my sister in law is married to my husbands ex wife brother. I have endured many years of being social with sporting events and my SIL parties. However this year has taken the cake, which now includes the ex participation at my inlaws. Parties or just plain get togethers. Xmas and now new years! Ihave opted to avoid these functions by either working or just plain acting
Ill Now SIL is asking my husband why he can't participate in being w his family. I feel bad but I don't want to be put in an awkward position anymore. I don't want to be my husband ex friend. I didn't like her before he married her and I surely don't respect her, therefore I don't want to "hang out w her" I am now developing anger and resentment w my MIL and SIL. I don't want to be selfish and have suggested my husband go byhimself to these functions, but he semi refuses. ( which means he is there until I get off work, which is usually late). Any suggestions to handle my drama?

How is this affecting the child?

Your only solution would be to agree to disagree. Set aside family time as you would with a custody agreement and trade alternate years etc for celebrations. Your child shouldn't have to witness the added stress of something the parents are trying to create.