No one was there to protect your dh and his brother. They suffered and continue to do so to this day.
This time it's different, your dc have you and dh to protect them, AND BIL and SIL. That changes this.
For a start you don't let that evil bastard away with so much as a dodgy LOOK in the direction of your children. Your dh may be fearful of this old man, but you are not, and who cares about the trouble? Your dc are being abused/insulted and that isn't going to continue, not on your watch, not ever. All your h has to do is back you, and BIL and SIL. You are not asking the boys to speak up, they are not strong enough to stand up to their father, but you weren't brought up to be degraded, and neither will your children be.
The only meeting you need to arrange is between you and dh, his brother and SIL. to support each other in your decisions to back away. Mil needs to understand that you are ghave I got to put your children's health above her H behaviour, and if she chooses to turn a blind eye to what he did to her sons, and now what he's done to your do, and even to her own self, that there are going to be consequences. She alone is responsible for the choices she makes. She's welcome to visit you, whenever she wants to, but fil won't be. Not now, not ever.
If sh e can't prioritise herself, her sons and grandchildren, then she will have to live with that choice.
Brutal, perhaps, but this is the next generation being abused, on her fecking watch, so if not now, when does she expect the fallout to come?