DW,
You could also suggest to MIL that she stop by here and visit us at SR. Many of us are moms, and we understand what she is going thru. It might be easier for you to realize that she's not doing it TO you, she's just doing it - that MOM thing that many of us did to rescue our kids from themselves.
You're dealing with it from a spouse perspective, she's dealing with it from a mom perspective. It's totally different, but it doesn't mean one person's pain is less than the others.
Bottom line? You have to set your boundaries and do what's best for YOU and take care of your children, because they are still CHILDREN and need your strength and help. Your AH and his mom will have to do their own things too.
Hugs, from one who has been wife AND mom in this mess,
Cats