OP again...just found out that the inlaws were free tonight and available to babysit...that makes me even more mad that SIL didn't go to them! I don't understand why it was made out to be a situation where they had no one else that could watch their kids....and again, why is their night out a higher priority than mine?
Anyway, I'm very tired as I barely slept last night. I'm going to have a few glasses of wine and call it an early night. H asked me what I was cooking for dinner. I laughed and said he could order pizza.
I'm sorry, OP, but you do seem to be overreacting to this particular situation. That said, if this is a pattern with your H and his family, you need to address that with him, perhaps in counseling. He is your problem. Maybe your SIL is not a favorite person of yours, but you are putting too much blame on her in this situation. Why be mad at her that your inlaws are free to babysit? Not her fault. You don't know why she didn't ask them. You also said that your SIL offered to not have you sit once you had an issue with it. She offered to handle her fundraiser herself. It's your husband who is making problems for you.
As for why her night out is a higher priority than yours...well, it isn't. But the issue is that your husband thinks it is. Obviously, if you didn't already have an issue with him, you would not have a problem moving a date night for your SIL's fundraiser.
You have a lot of anger but it is directed at the wrong person. Stop looking at your SIL and deal with your husband and the issues in your marriage.