Uninvited holiday guests

Why does this bother you so much?
That's not inflammatory - have a think.

It would discombobulate me a lot - but I'm autistic. So I'd take a deep breath and say
It's OK that plans are changing and due to something outside of my control. You can deal with this.
My kids are my priority. I will focus on them, their sleep needs etc.
I won't beat myself up if I can't conform to how I perceive a normal holiday evening should go.
I might feel uncomfortable being around new people. It's 1 night. My dh has my back and I can be a bit withdrawn if I need to
These changes haven't been made because bil is thoughtless, spiteful, trying to take control and make me feel uncomfortable - this is something NT people do. It's a nice thing.
I have to be open to enjoying something new.

One night of two weeks, and they're going to cook for you all? Complete non-issue and you sound like a fun sponge. If you don't want to join in just put the kids to bed and then go to bed yourself.

As for their camper van, if there is some rule that they can't park up in the Airbnb parking, or there isn't space, then they can literally park in the nearest parking spot and sleep in it there, so long as they don't set up a 'pitch' around it.

They don't need to go off to a campsite.

Who booked the Airbnb? I would check carefully in T&C that there was no an issue with this. It’s my main concern.
Otherwise they might be ok, just joining effort rather then planning a wild night.
However it seems that your BIL and his girlfriend tend to do things in their own way without asking others so I would dig out my assertiveness for this trip.

They’re not uninvited guests.

They’re not crashing your whole holiday, just one night of socialising. They’re not even asking to bunk up in your accommodation.

It’s not all about you, it’s a holiday with other people too. If you don’t want to join them at the BBQ make your excuses, put your kids to bed and then go to bed yourself.

You’re arriving in the evening at 6pm and they’re only staying one night? You’ll hardly see them. Let it go.

It's only one night. It's a group holiday, it doesn't centre all around what you want.

One night and they’re going to cook, I also think this is fine.

Nobody has suggested for a moment it's going to turn into a 'girls holiday'. Two girls, whom presumably are travelling in a camper and will be at the same place on the first night, are going to join for one evening.

OP comes across as a proper fun-sponge.

First not soon

Well she shouldn't have invited anyone to a group holiday without checking with the others soon. If she wanted to go on a girls holiday she should have arranged that another time

Just ask them to be quiet so that you can get the kids to bed. They might be nice people to have a chat with in the evening- how nice for the girlfriend that her friends will be in the same place, of course she wants to see them it's her holiday too. Are you jealous of single women being around your husband or something?

For one night, I would just put up with it. I also hate it when people invite themselves to things.

This is why I don’t do this type of holiday.
I’m also always very vague about where I’m going on holiday. I’ve got 2 holidays booked and when people have asked I am not disclosing the names of the villas etc. Neither do I disclose which travel company I have booked with.

Your not the only one going on holiday, moaning for moaning sake really.

A bit precious tbh. Anyway, you don't have to socialise with them, you can sit inside and cook for yourselves if you prefer.

You have reminded me why I never go on this type of holiday set up!

Who booked the Airbnb? If you have access to the booking just message and ask if it’s ok.

They aren't uninvited though- someone has invited them

They're not 'uninvited' if the BIL's girlfriend has invited them.

Suck it up, buttercup.

I would be annoyed and I'd speak to your BIL/'SIL' about this (assuming his GF is coming) and let them know you're not happy. Are these friends already there? You might find you're all travelling on the same ferry!
But let's assume it's your SIL who's told them she'll be on holiday and they've arranged to meet up for one night as its close by....I wouldn't like it but it might cause more problems to say no...unless of course the BBQ and the campervan will cause an issue. If the booking is in your name I'd be checking the T&Cs very carefully. Maybe there's a charge for pitching in which case I'd forward it....or I'd look for somewhere local and say if BIL and SIL want to meet up they can go to XYZ campsite.
Don't blame the friends. Chances are they know your PIL too.

YANBU if it was more than one night. But I think, on balance I'd try to just make the most of it. You may really like their company. Or you may want to concentrate on settling the kids the first night anyway.

I don't think one night takes the piss. If it was multiple nights, then I think it deserves a conversation and agreement.

That's the trouble with holidaying with other people- you're not in complete control of plans.
There are inevitably going to be compromises and differences in expectations.

This is what you say to your bil. He needs to check with the property host regarding parking the camper van overnight. If it is not allowed, then he is responsible for finding a campsite to park in overnight.

They're not staying in your property so there won't be any inconvenience to you.

I hope whoever has booked the accommodation has checked that it’s ok to park a camper van on the property, as I doubt they will agree to it. Where we’ve stayed, it has been made clear how many people are allowed

They can park somewhere else if it's a problem. Honestly it's literally just an evening and the reason why you can't articulate why it won't work is because there isn't a legitimate reason apart from you not liking the sound of it.

Get in, feed kids, put them to bed, eat food that someone else has made, either have a nice time or eat and go to bed.

I hope whoever has booked the accommodation has checked that it’s ok to park a camper van on the property, as I doubt they will agree to it. Where we’ve stayed, it has been made clear how many people are allowed.

Can’t see the issue . It is one night on the property offering to cook .

you never know you might have a nice time ?