C., I know its not easy and I'm going to give advice in the most polite way that I can because I know you're already stressed to the max.
My mom was in a similar situation to you. My brother and sister are both lacking in parenting skills sometimes and because of this, my mom tends to take over and "help" then feel angry and hurt because she feels taken advantage of. The more she helps, the less they do. She babysits both of their kids, takes them to all their doctor appointments, pick up and drop off for school, meetings with teachers, buys groceries, school supplies, clothing, etc....and on and on and on. The more she does...the more they LET her do. I once asked my brother if his son wanted some strawberry ice cream and he looked at my mom and said "I don't know, does he".
As the person who lives away from home, with my own little ones that I CARE FOR ON MY OWN, and views everything from the outside looking in, I can only tell you that you need to follow your younger childrens' lead and just stop. I know its gonna be hard, because as my mom says, she's doing it for the sake of the grandchid. But you have to realize sooner rather than later that the more you do "for the child's sake" the more your own child will take advantage.
I think the saddest moment for me was when my hubby and I announced that we were expecting our first child, my dad looked at me with a smile and then said to my mom "FINALLY, I'll get to be an ACTUAL grandparent instead of a parent stand in". I only wish my brother and sister had been there to hear it and see how this effects EVERYONE else.