Sis said no to plus one at Xmas?

Well that’s a you problem. My Waitrose, Ocado & Cook orders are editable until a couple of days before.

So you would invite someone because you feel sorry for them? Or because you want to spend time with them?

It matters. Or maybe other people don’t feel patronised by an invitation which is made because the host pities them.

How nice of you, that your crystal ball works well and you know whats going to happen on Christmas Eve.

Yeah well I do, I don’t feel comfortable with strangers having dinner in my home and I’m not putting myself out on Xmas day, I do that all year

Just think how this thread would have gone if the sister had posted.

I host Christmas every year, mainly because Sis is widowed, and it gives the kids time with their cousins.
DH is moaning we never see his family on Christmas Day, we do see them on Boxing Day, and take food with us. Boxing Day is chilled at the ILs.
We don't really have space for them and sis, I feel for sis on her own with 2 young adult kids.
Now Sis has asked to bring a friend, not even a partner, just a friend.
AIBU to say no?

No, you aren’t entitled to invite plus ones to other peoples homes or events without permission. Irrespective if it’s Christmas or not.

I'd just stay home and not go to the lunch portion of the day. Tell everyone they are welcome at your place for the dinner portion.

That is true, it would have been different, but that scenario does not reflect the situation here at all. From the information we have, your new scenario is not what we might expect from the sister's side.

Sorry I am so long-winded, I find it difficult to write. This is spellchecked so much.

The assumptions here seem to leave out that the work and Christmas food and space provision is joint between the sisters: lunch at sister one's house, and tea (evening) at sister two's house. The sister's huge house suddenly has a space issue in this new scenario.

The 'not even a partner' line and the DH's family being left out does not reflect the OP's community 'everyone is welcome' vibe. The sister finds being asked a burden in this scenario, but our information is: 'My sis wasn’t bothered about that'.

This scenario has someone condescendingly putting up with their sister as a transactional matter for the kids. The polar opposite of the OP's relaxed, warm, social family.
The reality is both lovely sisters seem to assume the lunch sister had a say in the evening event guest list because it is a shared Christmas.

Feeling for a single parent of grown children is an odd word choice, implying pity of all things? The OP is not on her own at all. For a start, she has a lovely social family and an actual sister.
Mentioning the OP is single in this way provides the co-Christmas host with a credible invitation reason, risking the lunch host looking pretty shabby for not inviting the friend. Yet the sisters both seem lovely from what we know.

There are many different ways to approach Christmas, but I feel what op describes is not this.

quotes from op for context
"Food etc at both events and workload are shared between us."
"I don’t think the issue is that I didn’t ask first. My sis wasn’t bothered about that."
"Perhaps it’s a cultural thing but it’s certainly not unusual in the community I was brought up in. Never seen as 'entitled' but more an open, everyone’s welcome, social and kind thing to do. I think that’s why I’m surprised."

I personally feel we can trust the lovely real sister has reasons op would agree with. I can think of dozens of possibilities.

I’m in walking distance of 3 supermarkets so I’m fine, but you might use you ball to figure events between now and Christmas.😬

Supermarket isn't always the answer.

I'm trying to keep this light hearted but sometimes serious shit happens & sometimes serious blessings.

Life just isn't always predictable.

It’s all fun and games until the supply chain goes tits up and people can’t get enough ‘picky bits’ 🤣

It absolutely is not "the height of meanness" to suggest OP's friend volunteers if she's alone for Christmas lunch. Many volunteers love the experience of doing this.

We don't know that OP's friend would otherwise have to be alone, as we don't know her circumstances or if she's explored other options.

OP has written a cheque to be cashed on her sister's account here. By inviting someone else without checking, OP wanted to be generous, but she was not the person who would in fact be accommodating the extra guest, so that "gift" wasn't hers to give.

People value different things about Christmas day, and for some it's a precious day with just their closest people. If you're hosting yourself, great, invite whoever you want - but don't make that call for other people.