Ah. From your post it reads as though you invited BL as he’d just come out of a 6 yr relationship and so presumably you thought he would be alone. Then it turns out that (unknown to you) this was no longer the case.
It also reads as though BIL assumed he might bring her along, without actually first asking if it was okay. That would irk me, especially at Xmas. How does he know you have the space, that you haven’t made specific food plans? For all he knows, you might already be inviting your neighbours, and it’s enough. But he didn’t bother, he assumed.
Good manners costs nothing, he could have asked.
To make matters worse, when you raise an objection, your DH is not really being helpful.
I would definitely say to DH that in future, nobody gets to invite others without asking first, and you both have to agree (especially if you do most of the work).
You could phone BIL yourself and say, sorry you only have space for him. Or you could swallow it this time, and maybe he will decide to spend it with his girlfriend instead.
If he is invited again, be clear about the boundaries. Shouldn’t be necessary but apparently it is.