Bring your Mac and Cheese. Kill her with kindness. So, “I’m so glad you decided to bring Mac and Cheese too! It’s such a fun surprise! There are never any leftovers since everyone always gobbles mine up and it’ll be so great to try a new recipe and get to take some of my own stuff home. I’m always looking for ways to improve and yours looks so different. I’ll have to get some pointers from you. This is so cool! This year there is enough for people to try both and then get to take some home. It is my favorite dish, after all. Yours looks like it’ll reheat well too! And it’s nice to have less booze! Now that we know you’re open to cooking things will have to have you bring more tasty recipes to try!”

She’ll embarrass herself by doing this.

I’d also ask a trusted friend to pull your brother aside and say something to the effect of “I’m so sorry about the Mac and Cheese. I didn’t realize your girlfriend didn’t have the sign up sheet ahead of time. I hope she wasn’t too embarrassed that she brought a duplicate and doesn’t feel excluded. I’ll make sure she knows how to sign up correctly next year. Thank goodness your sister was so gracious about it even though hers is such a favorite. Strange that she didn’t know after three years. It wasn’t bad though, definitely not a terrible first attempt for someone who has never cooked for us before.”