You know for the record, we don't know if SIL did actually say she would prefer to not bring the mac & cheese. We are getting the story second hand. OP saw she brought a box and knew the mother had asked SIL to bring it, but we don't know exactly how the exchange went down. Maybe the mother wasn't clear she wanted it made, and SIL thought it was a back-up, just in case, have on hand item, or maybe the mother asked for it and didn't hear or understand SIL saying she'd rather bring a salad as mac & cheese doesn't transport well. Everyone wants to blame SIL as so rude, but honestly it was pretty darn obnoxious for the "host" to request it of someone for a potluck. People are supposed to get to choose what they want to bring. So the only absolutely known rudeness was on the mother, any perceived rudeness of SIL is just that: perceived.
I just don't see "I don't know how" as a reasonable counter to a request to do something easy. Making mac and cheese isn't the same as giving a home perm or repairing brakes.
Honestly mac & cheese costs less then $2 and takes 10-15 minutes to make, but it doesn't keep well. Clearly the SIL wanted to bring green salad (and she did, fully made). If mac & cheese is so gosh darn easy to make then what is the big issue with SIL bringing the box unmade? If its easy for her to make and transport, its easy for the host to make in her own kitchen no transport required.
Regarding the bolded above, to be fair, we really don't know that either since we weren't privvy to the actual conversation between Mother & SIL.
What if SIL said, "hey what can I bring?" and Mom said, "mac & cheese for the kiddies".
Then Mom's not rude.
And this is the exact reason I don't ask that question anymore. Once I asked that question and the host gave me a specific assignment for something that I wasn't all that keen on making. But hey, I asked. So I brought what she asked for. Now I don't say, "what can I bring?". I say, "I can bring xyz dish" or "I can bring an appetizer" or something like that.
A lot of speculation going on in this thread (me included) and until the OP comes back to clarify, I'm not sure we can draw any real conclusions. My impression
(as I said way earlier on in this thread) is that it was a P/A move on SIL's part because bringing an unmade box just seems so much weirder than just not bringing mac & cheese in any form at all. Whether my impression is correct or not depends entirely on facts which have not yet been revealed.