If I was asked to bring Mac & Cheese and decided to use Kraft I'd ask if it would be okay for me to show up early and fix it there. I can understand not wanting to prepare it ahead of time because it doesn't reheat well.
I think where the SIL messed up was 1) not arriving early enough to have it ready at meal time, and 2) handing the box to the host. That was bad.
If I'm asked to bring a veggie tray I'm not going to show up and hand the host a grocery bag containing a few celery stalks, some broccoli crowns, a package of cherry tomatoes, a whole cucumber, a bag of baby carrots, a can of olives, a tub of sour cream and an envelope of Ranch dressing mix. If you agree to provide a dish you provide it, not the ingredients for it.