Agreed-- IF anything straightens Jill out. More likely this would just be narc fuel that gives her excuses to preach how the evil world has twisted her beautiful child against her, woe is Jill, the single least sinful person in the world!
But yeah, any rebuking from an in-law would just create a way for her to interfere with the child's relationship.
Abusers will try to isolate their victim from others. "I don't like your mom/sister/friend/etc." An abusive narc mom could certainly do the same thing with a child's spouse if they aren't serving her purposes.
It'll be a bit trickier for Jill, since she can't exactly encourage her kids to divorce, or encourage her daughters to tell their headships to pound sand. More importantly, she can't express those feelings on social media. But she can still insert herself into those relationships, and impose herself. Showing up at Nurie's with a broken leg to be waited on right after her first child was born comes to mind, IIRC.
If your loved one is in an abusive relationship, it's important to keep the lines of communication open. Don't confront your loved one and make them feel like "it's him or me" because that might help the abuser isolate your loved one further. It's better to maintain yourself as a safe option for your loved one to turn to for support when they are ready.
It's the same if your spouse is still involved with an abusive parent. Contributing to in-law conflict only causes more strife. I don't mean kowtow to the monster MIL and enable her bullshit, just don't let her make YOU into the monster, because that might just make it harder for your spouse to get disentangled.
Hopefully the Rodlets will find spouses and friends that can do this for them.