POF, I totally agree with you and with the other PP.
You have no control over how long the meet will last, and if it goes over, you are the one ending up doing the Big Scramble to make it all work - to get the rest of the family to Sally's Big Event. You.
If MIL comes along and based on past history, you are the one who will end up dealing with her. You.
If I were in your shoes, I would decline to go at all, staying home with your kids and making it a Mom's Weekend With Her Kids. Which, ironically, you would be giving as a gift to your MIL - Mom's Weekend with HER Kids.
Going up Saturday would be my own second choice, but if you are OK with it - then run with it. My concern for that is that, immediately upon arrival, MIL is placed on your hands while DH and SIL go prancing to the party.