This comes from a BM who horribly misses my exH's family, they've been family for nearly 20 years.I stepped aside to make my ex's gf more comfortable though I'm invited to family functions to bring the kids (exh did a lot of damage to his own family when we separated through some horrible mistreatment of his parents kindness and as far as I've heard never apologized)
I'm close with several of the cousins, MIL (both his bio and SM) but now wish I hadn't backed away from that side of the family as exh doesn't take the kids to see them and my kids are distraught and missing them (grandpa and grandma's health has taken a very bad turn for the worse both of them in a week)
I had to inform exH of this and I can't imagine how much it must hurt him to be told his parents aren't well from his ex.
I don't do any of this communication to cause harm or butt in but because it matters for my kids.

I wish I had an answer to this one, because that is my family. Especially my dad. He insists on maintaining a friendship with my very verbally and emotionally abusive ex husband. It doesn't matter what is done to me in front of my dad, what I show him has been said, nothing. He tells me that his friendship with doucheyX has nothing to do with my marriage ending. The thing that kills me is that my SO feels very unwelcome in my family now because of this. My family's insistence on maintaining a relationship with my ex is going to ultimately sever my relationship with my own family. Which is probably my ex's plan all along....

On the bright side, my mom and dad actually offered to take FSS during the overnight visit my kids were going to have! At least they are finally starting to open their arms to him

Every family is different. My generation has been the first to experience divorce in my extended families, and our families way of thinking of things is that when someone marries, their spouse becomes part of the family. And once that person is part of the family, when they divorce, that doesn't mean the ex spouse is going to be shunned. This is really good for the children, because they don't have to witness their parent being treated disrespectfully, and it means fewer relationships are ending and changing. Not all families feel that way and are able to do it that way, and it would be a lot harder if one of the ex spouses was a drug addict or something like that.

My ex husband's family has experienced more divorce, but he had cousins who divorced and the ex still came to the family reunions....

Of course the new spouse after the remarriage is always treated with proper respect too.... no one ever puts that person down or disrespects that person.... they just don't feel a need to end their own relationship with the ex just because the marriage ended.

I still maintain contact with one sister in law after the divorce. I was married to her brother for 16 years, she and I had developed a close friendship.... the way I see it, why should I end my friendship with her just because her brother wronged me?

If they are disrespecting, undermining you, of course address the disrespectful behavior... but you really can't dictate to adults with whom they can maintain relationships. Them choosing to maintain a relationship they value is not about you, unless they are trying to break up you and DH..... and how much influence he gives them is up to him.