AIBU to feel like my holiday has been ruined before we’ve even gone?

Honeslty I would talk to your DH and if he agrees I would drop out of the holiday. Send the kids of with DH and the grandparents and just do it on the basis that you are giving DN your spot on the trip. You get a nice peaceful week to yourself! But maybe Im way selfish

I’ve been told not to say anything as it would make things worse.

Confused...sounds like somethings going on you haven't been told about.

Why will it make things worse?

They sound absolutely ridiculous. We have a holiday cottage full to capacity. I know let's jemmy in another body so someone has to sleep outside! Muppets.

I'm even more confused now. This child goes on family trips with her own family but has been invited along on your family trip and is being taken out of school for it and no consideration has been given towards you.

Does the child get on well or is close in age with your kids? Is she having some kind of issue with her own family that they want her on holidays without them for a week, like surely the rest of her siblings will be wondering why they don't get to skip school and go on an extra holiday with Granny. Sounds spoilt.

As PP have mentioned I would say..that doesn't work for us and the holiday let has said no extra people or campervan. Your MIL is very self centred and inconsiderate of both you and her son.

Then the granny should have been honest up front instead of making the OP think she wanted to spend more time with OPers children.

That way it would have saved the OP being in the situation she is now.

Very underhand way of going about it.

It’s your holiday. Stop being a jellyfish & stand up for what is best for you & your family.
The PILs & SIL are CFs who will push you beyond your limits, why are you & your DH so passive with them. They are totally unconcerned about upsetting you, why are you so afraid of upsetting them???
Just tell them the whole thing is cancelled & go on your own. If needs be contact the holiday home owner to confirm that extra guests are not permitted, nor is it a site suitable for camper vans (although this information is probably already in the t&c of the booking confirmation).

OP, just do it for yourself and your children: you must absolutely tell PILs that the niece is not coming; do not say you won’t, or make any other suggestions to resolve it like talking to the child’s mother - is not on you. It’s particularly outrageous that they’ve had hoisted this on you at the last minute. You’ll regret it every second of the next week if you don’t act now.

If at all financially feasible I'd book something else for just you and your children.

If your in-laws are upset, so what? They obviously don't care about upsetting you.

Edited because I've just seen that you paid? Is that for the full thing? If so, tell them she's not coming and they can book something else if they're unhappy.

That's probably why you weren't told until today that she was going - because now they think you'll feel it's too late and that you'd be upsetting this child by saying she can't come. But it's actually your MIL and her partner who have let this child down because they are the ones who invited her when you had already told them you couldn't cope with additional guests.

I get that it's stressful to tell a child no, but by allowing yourself to be steamrolled because you won't stand by your boundaries, you are actually encouraging your MIL to cross your boundaries.

I have read all your posts OP and apologies if I've missed this, but will MIL/SIL be contributing any money for DN to join the holiday or are they assuming you'll cover it (you mentioned you paid)? Not just towards accommodation but there other costs. Food, drinks, activities, etc.

I can totally understand your frustration and in your shoes, with all the context you've given, I think you would be well within your rights to say no to DN joining this holiday. If PIL want to take DN away that's up to them, but to invite her on your holiday (that you organised and paid for) is completely out of order.

YANBU to be upset and disappointed that your PIL have behaved this way, particularly when you are paying for the holiday. You are being unreasonable in not taking action. Why are you asking the trouble makers if you can call the other girl's parents? Just call them and say there has been a mistake and their child isn't invited. If it's too late to cancel, I would actually uninvite the MIL and her partner as well. But then, I'm at the 'fuck it' stage of life. You don't need to be taking everyone else into consideration when no one is considering you. You're also not well enough to be caring for someone else's child, particularly when they have violent behavioural issues, and it's ok to say that out loud.

You are acting as if you have no say in this. Nor your husband.

Another vote for cancel and then rebook without telling them. Or tell them you've cancelled and then go by yourselves quietly .

You’ve said your children are counting down the days but do they know that their aggressive older cousin is now coming? I’d cut your losses and go somewhere else, tell PIL clearly that this was an opportunity for them to spend some quality time with your kids and as they clearly don’t want to do that you’ll focus on your own holiday.

Btw @Heathercost I have POTS too (and ME)
You absolutely need the hols and the rest. And a nice time with your dcs with no added stress.

Dont let anyone disturb that (or as little as possible)

I’m Wondering why PIL don’t want you to speak to child’s mum….

Is it because they kept the fact you wanted a hols with them but wo the child for a change very silent and the mum would be horrified?
Is it because the mum has been pushing for the child to go?
Does the mum have a history of creating problems or maybe theyre the golden child/theyre playing the martyr card?

Just tell them you’ve given it some thought and you would prefer to go away as a family as you don’t want the extra responsibility, if they’ve paid anythung just pay back what they are owed.

Who did you book with?

Honestly if they really care so little for you and your family and health I’d look at what it would cost to cancel (you’d loose a deposit?) and rebook somewhere else, if that’s possible?

You get one week with your kids, your health is poor, and you’ve made your feelings known. Boundaries.

I’d tell PIL that child is sleeping in the camper an as she wasn’t planned for the house. MIL or FIL or both can sleep there with her.
THEY will be fully responsible for tge child. Sorting out breakfast, snacks, activities for her. Esp as they are different age so not interested by the same thing.
You won’t see PIL that much but then they created the situation and it’s betyer than either not going or dealing with child all week.

Then as you said, refuse any holidays together

“I’m sorry MIL Niece can’t come on our family holiday, as this doesn’t work for us. Fully understand if you want to drop out”

You can’t be a doormat and sacrifice your children’s holiday, even if you and DH don’t like putting your feet down. Tell her now that they and niece are not coming anymore. Honestly, saying no then capitulating is worse than not saying no. They are taking the piss and you’re letting them! I wouldn’t have niece around my kids in any circumstances given she’s violent, messy and unmanageable. Say no. They need to learn to fear upsetting you more than they fear upsetting useless SIL. If you don’t you’re letting your kids down.

“the granny just wants all her grandchildren with her”

Well I’m afraid it’s not granny’s decision as to who goes on holiday with the op.

If you've paid for the whole thing then I would talk to DH about uninviting them if they won't leave the child at home. It's a bit trickier if you've split the cost