So, after talking to my husband, it turns out I was right. He did know what was going on, but neglected to discuss it with me. Everyone is going to his sister's house. But, since his sister lives 45 minutes away, he doesn't know if he's interested in going, after driving 600 miles home.
I don't fault his reasoning, and it would certainly be fine to spend the day at home just with my little family (most people's dream holiday, from what I can determine), but it would be nice to know about it! But, now I do.