Unexpected guest on Christmas Day

She wasn’t invited by the hosts, who are complete strangers to her. She got a second hand invitation from BIL, who has no authority to add to OP’s guest list. Any person of couth should realize that.

Showing up to spend several nights with no proactive invitation from the actual homeowners is tacky, classless behaviour.

You have embellished OP's comment. Regardless, the OP and DH had agreed to invite a single BIL over to stay, not BIL + 1 stranger to stay several nights.

Your comment "The only one with bad manners would be the one inviting, but later saying: you are only invited alone. I am not family either, I am only the wife of your brother but I don't want to see your girlfriend" is weird and delusional. So OP is 'only the wife'., is she? Wow, how dare the wife, joint home owner, mother and host feel no objection to agreed plans being run roughshod over.

And yes, the point about space still stands, at the dinner table, with food catering and everything.

I would uninvite BIL on the grounds that he hasn't rsvp'd in a polite, straightforward or timely manner. He sounds evasive and rude af, so that would be a no thank you from me.

I'd be fine with this for them visiting for the day and in that respect would feel its the more the merrier. But I would be much less cool about an adult I don't know staying overnight in my kids home and would decline that aspect. So I'd say yes for the day but not to stay over and let them do as they see fit with that. But equally it's up to you and if you don't want to feel like you're hosting then fair enough. It's not unreasonable it just wouldn't personally bother me for them to be there on the day.

I think it depends on the GFs circs though. My ex after a year of dating invited me to his sisters yearly Xmas do and I really enjoyed it having spent most Xmas’s alone. His family were def the ‘more the merrier type’ and it was just really nice to not be alone for a change and be part of the ‘real spirit’ of Xmas as it were without sounding cheesy 😊.

That said, if a new partner said his family didn’t like anyone other than close family/LT partners then I’d be fine with that also as I think it’s perfectly reasonable for people to prioritise the types of Christmases they want.

I'm an introvert, and I love a quiet family Christmas. So on the face of it, I should be first in line saying yanbu.

But no. I think yabu. Sorry! You say you don't know her. Surely this is a perfect opportunity to get to know her? There will be so much focus on everything else, that it's kind of a low pressure environment for her. And I find the idea that he wants to invite her because he gets single baffling. Why would he feel single. He has a girlfriend!

You may be right, maybe she wouldn't want to come. In which case she will denied the invitation anyway.

Off topic, but I’m intrigued about who you count as your DH’s family. Because you keep saying his family are invited and then “MIL as well”. Who’s the rest of the family if not MIL and BIL?

If it were me I'd ask them not to bring a plus one, as we're keeping it just family. But we look forward to meeting her next time.

How on earth is graciously accepting an invitation at face value low-class? It's a nice and normal invitation to make so why wouldn't it perfectly reasonable to accept it.

I don't get this gatekeeping Christmas as so precious it can't be shared with anyone unless you've known them a specific amount of time

Surely what your DBIL should have done is asked if he could bring his new squeeze not just make the assumption he can saying he might come and bring her. I would say no on that basis and tell him why

That’s made me feel really sad for your daughter. I’ve already posted but I hadn’t read that she’ll be staying Christmas Eve etc. I think I’ve changed my mind and that you should contact BIL and explain what you have on here to him, that you can’t wait to meet her in the New Year but that he needs to rethink his plans. Surely she’s going to say no to waking up Christmas morning at someone else’s house that she’s never met? This update made me think of all the posters that get ripped to shreds because they want their long term partner to move into their home. There are dozens of comments about how the children should come 1st, they should be able to walk around the house in their PJ’s, shouldn’t have to spare their space (even with men that they’ve met many times) and feel comfortable etc etc. Christmas is a special time and everyone slouches around a bit don’t they, eating too much, PJ’s until noon etc. I think use this to fuel your conversation with BIL. I think your husband is a bit weak for not agreeing with you, I get that he was put on the spot but he could be saying ‘in hindsight, I wonder how the kids will feel with a literal stranger watching them open their presents.’ Men.

Why is it OP's job 'to make BIL's Christmas'?

Where’s the goodwill toward the OP?

If the girlfriend had any character she wouldn’t accept a second hand invitation to stay in the home of strangers over Christmas. How low-class!!

Your BIL doesn't just get to decide to bring someone.

DH should tell him there isn't room.

I would not have a stranger staying overnight in my home at Christmas. No way.

YANBU.

And it’s only 12 Dec, you’ve got ages yet to sort things out, make BILs Christmas!

Absolutely no way would i have a stranger sleeping under my roof on Christmas Eve or Christmas night.

Nip this in the bud immediately. They can get a hotel or stay home.

Ive known this situation a lot and to me it seems quite normal. Christmases were always a juggle with different guests and it usually worked out well. It will probably be fine but if you are really worried about it then dont do it. Im sure it wont spoil the dynamic though. They are probably loved up and will enjoy going off for walks and things like that. Also she might become your SIL and not inviting her might not be a good way to start.

I think it's the fact that they will be staying over at OP's house for several days is the problem - It's not just Christmas lunch.

Not to stay over, no. Will there be lots of sex going on?!

It’s the men that are the problem here! Husband shouldn’t think you’re being difficult. And why can’t BIL just spend Christmas with his new partner in one of their homes? He could have just declined the invitation and said he was seeing someone. I don’t think Christmas Day is the right time to be introduced to a new partner. I mean it’s not the end of the world or anything but it seems a bit selfish, I wonder if the partner even wants to come, I wouldn’t want to meet anyone new on Christmas Day in their house, I would find it really awkward I would want a soft launch first - Christmas Day can be a bit stressful. I’m not sure what you can do now though if your husband it okay with it, I think you’ll have to suck it up if you don’t want to argue with him. Hopefully the new partner is really annoying to your husband, think opposing political opinions haha.

I wouldn’t like this either. Totally different ball game if they had been together a while and you’d met before. For me it’s the fact they’ve literally just met and you don’t know her. It has the potential to be extremely awkward and uncomfortable. He also has no idea how long this relationship will even last, she might be a flash in the pan. Why is she wanting to spend it with him and your family too, where are her family??

He should have asked rather than assumed.

It's not just a meal though - it's sleeping in OP's house for up to 4 days

I would say no. He clearly didn’t think it’s through, put it mildly. 🙂
You are going to have very kids oriented Christmas. if he wants to enjoy time with her he should change his plans and do something with her.