Your poor MIL!!! If I'm understanding this correctly, your parents planned this trip for themselves and your family, and invited your MIL to go too. Your SIL then invited herself and family along, although they are not included as part of the plans your parents are making; they are piggy-backing on to your parents' family vacation. Thin ice here, and I agree- you are gonna be the bad guy, regardless of who says what. This is the grim reality of WDW- it breaks families apart

(yes folks, that's a joke). All kidding aside, people really DO end up with hurt feelings over
Disney vacations- happened to me too, with my DSIS, which I won't get into but I will say that we are going to visit them for the 1st time in 2 years this weekend- and they only live 3hrs drive away.
I think someone needs to make sure your SIL (notice no "D" here?) knows that this is not YOUR trip or your MIL's trip to plan. Does she realize you are going because someone invited you to go, that it wasn't/isn't your vacation plan? If friends invited you to dinner, would your SIL tag along or intentionally show up at the same restaurant? That's what she's trying to do, essentially. I agree with PPs who suggest that you tell her that you are travelling with someone else, here is our itinerary, what is yours, perhaps we can have a pool day on xyz (which you already know will be a nonpark day). Make SURE she knows that you are not going to skip out on your parents' plans because SIL wants to visit you while you are on vacation. If she squwaks, tell her sorry, but trying to come visit us while we are on vacation isn't a good idea!! Also, TALK TO YOUR PARENTS!! Don't leave them in the dark. Let them know the pressure you are under, while making it clear that you don't want to change any plans they are making. Tell them your SIL thinks she might be in florida at the same time.
Ah, this is a crummy situation and I'm sorry you are stuck in it. DO let us know what your BIL has to say after you talk with him today. (And don't be surprised if he's angry that yuo are tempting his family to go on a vaca they cannot afford, or are hogging your MIL's time. SIL/BIL sound like an irrational family!)