I'm kind of curious about how the initial exchange went, because it's not clear if the SIL offered to bring stuff, or was told to bring stuff. That's an important distinction.
Something like "I'm thinking of next Saturday for the kids party, does that work for you?" "Sure, what should I bring" "Well, we've got lots of kids so how about mac and cheese?" gives a very different background than "The kids party is next Saturday. We've got lots of little ones there, so you need to bring mac and cheese."
In the first case, the SIL was definitely being rude. In the second case, it could be a case of her feeling railroaded into bringing a dish on command to a party she didn't get any say in planning and couldn't decline to attend.
It also occurs to me that Kraft dinner is a lousy potluck dish - it's not nearly as good when reheated, and the quick cook noodles do not handle being kept warm for long periods. Homemade mac and cheese is more sturdy, but I don't think I'd make that for a batch of small kids, unless I knew that they'd appreciate the homemade stuff over the boxed stuff. Plus, good home-made mac and cheese for a big group can get pricey for a potluck dish, if you start with good quality cheese.