Ahhh jeez, tuffy. You can't catch a break. You work so hard on you and then another set of issues enters your life.
Side note: I think we're related. Well up until you mentioned the 3 kids of your sister, I thought your sister was my wife.
Anywho, my MIL is an enabler and so is my FIL. My AW is 100% spoiled brat and never needed to do much to run her life. She never really grew up. I know this is slightly different but I've detached from my inlaws in a big way. It's not up to me to decide what they do or don't do for their daughter, my AW. Also, just like our alcoholic's, we need to let them live their life and grow up.