we have it both ways...
In my family, it's usually 'just family', and we all feel like it's a group decision if someone outside is invited, no matter who is hosting. We don't look at it like "it's my house/their house, they can invite who they want". We all look at it like it's *our family* christmas (where it's being held is irrelevant), and we'd all have to agree to something like that. I'd never in a million years invite my neighbor-friends to a family christmas I was hosting, and I know my brothers/wives or my mom/stepdad wouldn't either, or would ask the rest of us "hey, this person will be alone - can we include them?"
In dh's family, it's more like whoever is hosting has the say, and we're 'guests' there, the same way a neighbor/friend/extended relative would be a guest there. And that's ok too. There aren't any 'group' decisions with dh's family.
It's just different family dynamics. Neither is right or wrong.
But think about how you would feel if your MIL was the usual host of your christmas day, and you got there and they had a few other older couples over there too, who weren't too kid-friendly, and your kids didn't get grandma/pa attention, etc, because they were chatting w/ the adult friends.
With another family of 6, it's going to be more like a playdate than Christmas, and if that's what you want, that's ok. But just realize that's what you're setting up.