To expect parents in mid '70's to host Christmas with very young children?

Bullies will latch onto just about any difference, however trivial, and its not usual to 'explain' bullying because the explanation doesn’t lie in the victim but the bully.

But let's say OP is proud of sister being such an academic go-getter. Good for her. But why then does she think this anecdote is going to go some way to persuading us that sister should be kept away at xmas? I don't get it.

Can't you and your DH and your brother and DSIL help? If you have both hosted before you know what needs doing. I am 60 I used to have 24 people for Christmas but I had my Mum and my sister to help. I couldn't do it now with everyone pitching in. It might make your parents unhappy if they feel the are excluding your sister. Your sister might feel sad at being alone. Encourage your sister to help as well.

Ignore the people wilfully side tracking

OP has clearly spent her life ignoring anyone who suggests her sister isn't responsible for literally everything that goes wrong, including having a fractious baby, being violently bullied at school and not supported with it and, er, living in London and wishing to spend Christmas with her parents.

So don't worry.

So you don't want your ideal John Lewis family Christmas to be contaminated by a poor person?

I think offering to help your parents out to make the visit easier in advance might be the way to go here. Could you visit to help them with toddler proofing/ fitting stair gates / maybe some of the cooking that can be done in advance? Would arranging for a cleaner to visit before/ after your sister help?

Ignore the people wilfully side tracking and trying to make out your sister is the poor downtrodden hard-done-by pauper in a tiny council flat struggling to afford her children and likely to be alone and miserable at Christmas, it is very clear thats not the case.

Unfortunately all you can do is encourage and support your parents in saying 'no' to her when she attempts to invite herself for Christmas. They have a standing invite annually at either yours or your brothers homes. When they feel that hosting your sister is too much they can decline her self-invite and come and stay with you or DB.

You can't do more than that. Telling your sister she is being unreasonable to your parents will result in more family discord and her trying to justify to herself that she is not being unreasonable or rude or unfair, and your parents are likely to suffer the most from that.

Because 'different' has a whole range of meanings... OP would have been equally criticised for 'othering' her sister. Instead, she saw that her sister wanted to learn, the boys, it was mostly boys, she said, disrupted that so her sister remonstrated with them. Many kids are frustrated by their learning being interrupted, it doesn't mean they're 'different'; few are brave enough to confront the disrupters, who are usually an annoying minority in my experience. OP is admiring of her sister's drive and ambition throughout.

I'm a single parent of two children, my parents are in their late 80s and still insist on us going there. They don't like coming to mine, as they are more comfortable at theirs. I can cook (with my very able dad, who enjoys it), and wash up, and they get to see the grandkids. My brothers live in France and Scotland and if my parents didn't see us, they would be alone at Christmas. They would much rather be with their grandchildren. Sometimes I would like to be with friends instead, but that's not an option, and that's fine, as they won't be around for ever, and the more memories my kids have of them, the better. Mid-70s isn't even old!

Why didn't she just say 'she was happy at primary school, but at secondary school she was physically bullied by several boys in her class for being "different"'? (I'm sure OP didn't mean to make it sound as though she brought it on herself.)

I think this is such an odd thing to make a comment on.
I suggested an adult daughter could help with the cooking on Christmas Day.
You're saying this is impossible as she has two children with her.

There would be 3 able-bodied adults in the house.
Are you saying it's impossible for those children to be cared for?

In her own home, which is presumably set up to be safe for 2 small children, where she knows where everything is and can control the environment, is very different. SOME babies sleep a lot. Some babies cry a lot instead.

You seemed to suggest that anyone staying with elderly relatives for more than 48h was unreasonable.
I was pointing out that if your relatives life far / abroad, that's the only option bar not seeing them at all. I wasn't specifically referring to Christmas although it applies too.
Once money has been spent on travel, it's rarely realistic to book a hotel.

@Commonsense22 but this post isn't about a foreign travel situation. And even when everyone is in the UK, I would also suggest that staying with elderly relatives for more than 48hrs is a bit much.

You really are putting words/thoughts into @Birminghamx post. She says her sister was intolerant of the boys messing around in class, disrupting her learning. She states her parents and the school didn't deal with this, not unusual as we know, therefore her brother who was there at the school had to keep her safe. A loving brother and obvious thing to do, and the OP sounds sympathetic to her sister. Where does this 'golden child' narrative come from? OP has already said she was the lazy, unambitious one, and that she loves her sister. 🤷 So I really hope you're not a counsellor@MrsTerryPratchett

You seemed to suggest that anyone staying with elderly relatives for more than 48h was unreasonable.
I was pointing out that if your relatives life far / abroad, that's the only option bar not seeing them at all. I wasn't specifically referring to Christmas although it applies too.

Once money has been spent on travel, it's rarely realistic to book a hotel.

She is your sister and you should love her unconditional... Why exactly? @sandyhappypeople Just interested, is it the blood is thicker than water thing? And is that necessarily true - apart being from factually correct scientifically?

And what @sandyhappypeople says.

I suspect OP can't see it because it's incredibly pernicious and attractive to be the golden child. I know people who died in the role, so strong is the pull. All the time martyring themselves doing care and complaining about the black sheep who has boundaries.

It's actually easier to be the black sheep in the end because at least there's separation.

My sister was so keen on achieving she was totally intolerant of the boys (usually boys) who messed around in class. They reacted and for a term my brother had to see that she was safe leaving the school.

This is a really interesting couple of sentences. Really interesting. Because when a child starts to develop what you think is a PD, what you are looking for is trauma. The first thing identifiable as trauma in your story, is framed as her being intolerant and getting the consequences of that. Not this:

She started to have a really hard time at school. Her classroom was chaotic, and not managed well. She wanted to learn and tried to make her learning environment by tackling the disruptive students. They reacted with bullying so bad that she wasn't safe at school. My parents and the school didn't deal with it and my brother had to stop her being attacked.

I think you need to explore your feelings with a counsellor. Your script is so fixed in your head, and I think in your family, and you are all so fixed in your roles, I don't think you can actually see it. You get something from her being the fuck-up. Work out what that is. Normally it's status and normally that's because there's a paradigm of black sheep/golden child. That's typically formed by parents. You all talking about how terrible your sister is really suggests that you are subconsciously being the 'good one' to avoid the clear disapproval that comes from being the 'bad one'. For example, in this case, everyone talking about you behind your back, saying you're going to be a bad mother, even though you are a good one.

Counselling can pick this apart for you.

I don't understand your post @Commonsense22
I've not spent Christmas with my elderly parents for almost 20 years owing to distance and their health.

How does she manage at home on her own to cook?
Babies sleep a lot, they have chairs to lie in, there are two able grandparents to keep an eye on them.

I completely agree!

Would the sister agree? If she says otherwise, then who is right?! Surely only the sister can decide that!

Glad your parents live close enough for you to have that choice.
Ours live abroad and with travel costs, hotels are not a viable option.
However, we do do all the cooking etc when there for the set of parents who struggle.

My sister had a wonderful childhood,

Odd thing to say about someone else. Surely the only person who can judge how "wonderful" their childhood was, is the person themselves.

OP, if you are the golden child (which I suspect you may be), your sisters upbringing will be very different from yours, to say you grew up in the same household and she had a happy childhood doesn't mean an awful lot, when you grow into adolescence and adulthood you start to recognise that pattern for what it is (your family not thinking you are good enough for a start because of X,Y,Z), it is very much a damaging dynamic within a family and can lead to a lot of resentment in later life.

She is your sister and you should love her unconditionally, she is who she is and you shouldn't be trying to police the relationship she has with your parents, as a golden child you see that as your 'job', and your mum 100% encourages that, by confiding in you about your sister (but not speaking to her directly herself about it), and you and your mum secretly trying to diagnose your sister with a personality disorder, because you are so sure there is something wrong with her?? Lots of what you have said here fit the pattern.

You don't seem to see anything wrong in your actions because you firmly believe that everything you do is right and everything she does is wrong.

The fact that she disappears for a while after "absolutely nothing has happened"' in your words is quite telling, I'm sure she will have a very different perspective on those events, yet you are using it as another reason to bash her over the head with it.

The weirdest thing for me is that the parents (unless I've missed it) haven't even said they're unhappy with the current arrangement. They didn't even say they were unhappy with the previous Christmas the sister spent with them. OP seems to have decided they were too tired based on information she received from the parents' friends.

OP you could ask your parents whether they're happy to host your sister for 5 days, maybe they are looking forwards to it. Try asking them what their ideal Christmas would look like rather than trying to work it out for them?

Easy enough with a partner. Who do you suggest should have been holding her "fractious" 5 mo baby and supervising her 2 yo whilst she was in the kitchen cooking?