If you want to leave, do it! No one asked you if this plan was okay with you, and that's just rude and inconsiderate. Let them fend for themselves completely. If the kids are mad, they'll get over it. I'm sure your husband and SIL know how to dial a phone and order pizza.
This makes me really mad. You DO NOT have to put up with this kind of treatment. Your SIL is waaay out of bounds. Do you have a friend you could hang out with today? I wouldn't be coming home till 10 or 11 tonight, if at all.
Big HUGS to you.