I didn't realize you had 4 of your own kids, I was getting a little confused in the original post. How old are your boys? Maybe they feel she's like a sister and don't mind her there.
Wishful thinking.
anyway, it's a tough call and I'm sorry. I babysat for my sister when our kids were little. Upside was that I got paid, downside was that it wasn't much, definately cheaper then daycare, and she gave me a 1099, after I had filed my taxes!!! I had no idea she would do this. I think she got a huge tax break being a single mom by doing it, but .... all the same. I was stunned.
I hope you guys can work it out. Sounds like maybe you and sis need to hit Starbucks and have a pow wow.