Sis said no to plus one at Xmas?

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.

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

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.

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.😬

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'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.

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

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?

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

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

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.

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

Who are we to say? Well I don’t know about you, but I’m saying it on the basis of the OP’s own posts, where she says her adult children are sociable people who are happy to have her friend over for Christmas Day. No evidence there that she’s sitting there lonely and crying.

That’s who I am to say.

Maybe amongst all this open socialising your DSis values quality family time between siblings, kids and cousins

Yes. 'everyone welcome' would be sociable, kind and inclusive but you can't deny it changes the dynamic.

Invite your friend for the evening and have a lovely time.

We don't know.

Equally that might well be why the Sister prioritises Op and her young adult family at Christmas, rather than alternating with her husbands family.

There are so many dimensions,
their own kids might want to bring friends, foods not free,
space, we'd have struggled to get 8 into our last house, without doing it picnic style on the floor.
maybe they only have one loo

Not everyone has a John Lewis house with tons of space

So you think it’s terribly mean to leave anyone alone at Christmas, but it’s also “the height of meanness” to suggest someone could volunteer somewhere on Christmas Day? What about all the people who would be alone, or even out in the cold, without charities offering Christmas dinner options? Someone needs to manage all that. Isn’t it a bit selfish to get terribly mawkish over the idea of anyone being alone, but to call the suggestion that anyone facilitate an alternative as “mean”?

You also don’t have to rely on others to “save” you from being alone at Christmas. What’s to stop you hosting a lovely big lunch for all the single people you know? If being alone is so awful, take the initiative!

Not sure what ‘cultures like ours’ refers to.

I’m English and whereas I might invite someone out of duty, it would feel odd to do so; and it would feel odd to be invited by someone because they feel sorry for me.

Mind blowing I know but its almost like not everyone does things exactly the same as you, even more mind blowing to know thats allowed and perfectly ok.

I see people on mumsnet often say 'Christmas is about getting everyone together' but for many of us its NOT.

For us its about quiet time alone, I can and do go visit and see people any and everyday of the year. Christmas isn't some magic gathering day, its the opposite for us its a day for private time. A day when we don't have to put on a mask and pretend to blend despite how uncomfortable and overstimulating it is. This is pretty common for millions of people with neurodiversities who just want to relax.

I agree, me also. I feel hopeful it's from people from the US with a different cultural expression of hospitality. While hosting may be a burden, in cultures like ours it often ends up a net gain that increases happiness, improves reputations, and strengthens community bonds rather than weakens them. And hosting someone a loved one trusts is lower risk.

No maybe she's lonely because her children are adults
.maybe she isn't who are we to say.

People get very set in their ways and rigid
.

Op could just as easily have said 'Sis I'll host Christmas and I'm going to invite my lonely pal'

We have no idea what other constraints the Sister is under, maybe her husband is fed up of his wife always having her sister at Christmas.
But the Sister feels for Op since she's widowed with 2 kids, I don't think that hole in a family ever really goes away.

I don’t have a partner either. I don’t invite strangers without asking.

Also, OP has said her children are adults now. She’s not some poor woman trying to juggle work and childcare who needs a break.

Bung it in the freezer to deal with too much food than not enough.

Never had enough warning to do anything about my Christmas order.

I wouldn't do it to look like I was a good person and I find that a bit of a strange thing to say. I would do it because I wouldn't want someone sat at home feeling like nobody in the world was bothered about them on that particular day when all the emphasis is on enjoying yourself and being with others. It might work out it might not but it would be an experience for all concerned - including the guest.There is one Christmas Day every year you know so there are plenty more if it doesn't work out particularly well.

So are mine but all orders are updatable, some until a couple of days before.

If yours is not that’s on you. What happens if part of the family go down with chickenpox?