L.A.
30 years? What a waste of energy and time.. And this all revolves around one person.
Your husband (also SIL) need to man up and let MIL know they will no longer stand for this.. To bring in a new generation of children into this mess, is like a sin.
This all should have been settled decades ago. IF your husband and his sister are not willing to put their foot down.. your children will be the ones to suffer. I also think they owe, FIL apologies, that they never took care of this. He deserves his own peaceful time with his children.. Enough is enough.. They need t sit Mama down and tell her they will no longer follow these selfish requests. And then they need to JUST do it.. MIL will then see what she has brought onto herself.
MIL has emotional issues. To devote this much negative attention on herself, shows she has many emotional issues.. Do not continue to "feed the beast".