Sis said no to plus one at Xmas?

No one is saying don't invite friends to yours for Christmas not even the OPs sister but it's completely unreasonable to invite them to someone else's house without first checking.

I can understand their point of view - this is someone they have never met, there is bound to be that sense of reserve and slight awkwardness in the presence of a stranger, and Christmas Day is not the best time for this.

On the other hand, it is rubbish being alone at Christmas - if it was me hosting I think I would trust that my family member's friend was a decent person and adopt a 'they will have to take us as they find us' mindset.

Im quite shocked by most of the responses to be honest . If I was the OP I think I would have the whole day at home, with her family and friend, and invite the relatives over in the evening.

You should have asked her first . Maybe she would have been more accommodating instead of thinking you were cheeky . But, I think she is a bit mean regardless. And to imply your friend is welcome at yours is a bit patronising. Holy cow !

Agreed, they’re being kind to have you and your children, but a stranger brings a different vibe and it’s their home. You shouldn’t have offered someone else’s home.

I would bet a lot that this “friend” is someone the OP has just started dating

I’ve had friends of my mum come to us on Christmas Day who were complete strangers beforehand. I trusted her not to be friends with a mad axe murderer and welcomed them to our house. Each time, we had a lovely time and it was nice to think thst we’d helped an old man to have a nice family Christmas rather than sitting in his own eating a ready meal.

I feel that I should be able to bring a plus one, even if it’s not a partner? I know I should have asked first but isn’t that a bit unreasonable of them?

Why would you feel this was reasonable?
You didn't even ask which might have changed things, you just think you should be able to with no discussion with the host?

You should have asked the host first. You have put yourself and your friend in an awkward situation. I would not have someone on their own on Christmas day if they didn't want to be. Not a lot of goodwill to all men on this site.

Does anyone aside from you have any relationship with this friend? Even met them before? Do you have kids?

This. Plus, a friend is different to a partner who might stick around for a while, and who they will want to get to know.

You're unreasonable to assume this was OK.

And other family?

are you local to the family you’re having lunch with? Or do you all bundle in to cars post lunch and head to yours?

So your family have met this friend and still don’t them then around. Fair enough. Respect that. Or are you genuinely considering forcing the issue?

Funnily enough I had a phone call last night asking if we could fit in an extra for Xmas day=isnt the only answer “yes of course”. I trust my relative’s judgement that the people they choose to spend time with will be decent people, so whyever not?

However I do appreciate being asked ahead of time.

It could be the Queen you dont invite someone to someone's house without checking first even most kods I know get this let alone a grown adult

How on earth is this so hard to understand and what is the +1 business it isn't a wedding

You have children?

Op, it’s a nice thought to include your friend, but no, you are not entitled to a plus one.

I invited a neighbour to mine for Christmas last year but I checked with my in laws first (well, got dh to check because they would be more comfortable saying no to him and I didn’t want to spoil their day).

Yes it changed the dynamic a bit but for one year, none of us minded (in laws aren’t traditionalists Christmas).

I have said yanbu because whilst your offer is lovely and kind, and in their shoes I’d like to think id say Yes, they are not unreasonable to say no.

I might say to them, “I’m ever so sorry - I didn’t think you’d mind and I’ve already said to lonely friend of course she’s welcome to spend Christmas with me. That being the case I think I’ll have to do Xmas dinner at my home this year - you’re all still welcome back to mine later in the day and I’d love to see you! Especially if you bring turkey leftovers! Although since my friend will still be there you might prefer not to. In that’s case perhaps we can meet up on 26th or 27th? I am sorry I’ve messed it up this year!”

If they've met them and don't want them there, I'd be wondering if the friend is in some way objectionable to the host.

Christmas is the one time of the year that everyone is supposed to be a bit nicer. You know, extend the Christmas cheer a bit.

They are your friend and you have asked. They want to keep it to "family".

Nothing you can do, but I feel for your friend.

One of why DC's friends was alone last year and I asked them to come here, and I have just asked them again.

Your sister has decided to host based on expected numbers and guests themselves I wouldn’t have asked my sister to then host someone she doesn’t or barely knows.

If I wanted to host my friend I’d host them myself.

Grandchild who will be there, yes.
She’s also being brought up with these kind of values