If I were you, I'd take pictures of your house and send to your husband. Pack a bag for yourself and your littles and go stay with gramps and go back and forth to grandma for a few days. Tell MIL she needs to say with your children since you are caring for her parents and you can't do both.

Tell your oldest children that now they are in charge. Good luck. And give them MIL's number if they need help. Then turn off your phone and let them deal with the consequences of being such total jerks.

People only take advantage of you to the extend you let them. You need a break. Give yourself one. The dying grandma and the immobile gramps as well as the littles take priority over the rest of the family. Your MIL will b*tch and whine but too bad.

And honestly, I think your kids deserved what you said.