Both is good. If you’re poly, you can say that. By bringing a different date from your spouse, you are essentially outing yourself as poly already, you should expect questions if the group you’re doing this with doesn’t know, especially if it’s family you’re close to (I’m assuming they are at least in steady contact, given that they’re having family parties).

I can’t really imagine one of my siblings bringing someone other than their spouse to a family party, but I’d have some questions for sure. I also love and trust my siblings, so if they told me their relationship was open, I’d believe them and fully accept their polyamory. I’d probably be confused that they wanted to come out like that, but I wouldn’t have an issue with it. Once enough time had passed for them to be secure that I really accepted them, I’d probably ask why, but I wouldn’t be mad no matter what they said, even if they didn’t want to explain it.

If they were cheating on their spouses, I would not accept that, unless they were in a long term abusive situation and trying to escape. Otherwise, I would leave if it weren’t my house and ask them to leave if it were.