I'm glad I waded through the whole thread. When I read the OP I thought the SIL was rude, but now I think a miscommunication occurred.
To be perfectly honest, I had no idea that 'homemade mac-n-cheese' was different than 'boxed'. It's all made at 'home', right?
The concept of a gooey, lasagna-style dish of it with different types of ingredients was alien to me. I was in my early 30's before I ever tried it. Delish, btw.
So, to be fair, I might have been that clueless. And I might have reasoned on bringing the box cuz the boxed stuff doesn't taste so good if it cools down and keeping it warm is bit of a conundrum. I would have brought the milk, butter and pan though and asked to cook it. And probably not perceived the quizzical looks from my in-laws!
To illustrate my ignorance:
About 12 years or so ago, we had two couples over for dinner. Couple A, were our best friends and Couple B, people we really liked but didn't know as well. I'd roasted a couple of chickens, made mashed potatoes, green beans, homemade rolls and a Lindy's style cheesecake for dessert. Not a huge fan of roasted chicken and don't care for mashed potatoes but I felt it was a safe menu.
A few minutes into dinner, Phil (husband of couple A) asks, "Where is the gravy?" (Couple B's shoulders jerk and they snort but keep eating).
I responded, "I do not like gravy. So I didn't make it." Couple B is now outright giggling but dutifully plowing through their food.
Phil said, "You don't like gravy? But you can't have roast bird and mashed potatoes w/o gravy."
Phil's wife, Jenny, interjected: "Phil, shut up."
I smiled and then started to laugh, "I'm really sorry. I don't even know how to make gravy but I should have tried to figure it out. I promise next time I'll make you gravy!"
They liked everything else and we had beer and wine and good coffee to wash everything down so all was well but looking back I'm still a hair embarrassed about not even giving thought to gravy because I didn't like it. They forgave my cluelessness.
And this was before it was common to look things up on Google.