To not want to go on a 'surprise' forced holiday with MIL and 3 Nephews

Either

Suck it up and grit your teeth. It's only a couple of days. Look forward to when know kids are grown.

Or

Go down with a mysterious bug/ dog sitter has rabies etc.

Failing that

Get DH to talk to her and find an alternative that you all like.

Could you agree to go as day visitors only and arrange for a friend to look after your dog, or ask for a weekend just the 3 of you? You could say something like 'we've been thinking about your family tradition of the weekend away but it isn't fitting in with our family at the moment. We'd love to go away with you just the 3 of us or potentially come along for a day visit with sil/bil (providing booked destination is within an hour or two from home), however we won't be open to a whole weekend away with all the family this year. It's quite an overwhelming occasion - especially for [dogs name] and we don't have the capacity for it going forward.'
She can't dispute your situation. She can judge you for it, but it's not her right to dictate that you're going if you don't want to.

It’s tricky because it’s a present and a family occasion/new tradition. It sounds like you probably ought to go.

I do see where you are coming from though. Center Parcs is not my idea of fun, the DH sees it as no different from any other holiday camp holiday and would avoid

OP you can't say this without hurting your MIL's feelings - or probably can't. She seems invested in this and will probably be sad. She will also (correctly) guess you haven't enjoyed the previous trips and that she wasted money inviting you. That doesn't mean you're being unreasonable - of course you shouldn't go if you don't want to - but you do have to accept she will be hurt. Some conflict and disappointment is inevitable in families when individuals' interests don't align, I reckon.

There are two things I'd recommend you consider first.

1. In addition to your MIL, is there anyone else who will be disappointed by you and your DH not going? Will his sibling and nephews/ nieces care? And if any of these family bonds matter to you and DH, is it worth setting feelings aside for this limited amount of time once a year for the sake of family?
2. Is there any possibility, however remote, that you and DH will have your own DCs one day? If so, might you end up feeling isolated with that if you distance yourself from family things now? are you thinking too short-term?

Neither of these qus will necessarily change the outcome of the equation for you.If you and DH think about these and still come to the conclusion you don't want to go then it's perfectly reasonable to express that and hold the boundary (in my view).

Centre parcs has a spa and entertainment of stuff adults can do. Unless they are unpleasant people I would go for the weekend and try and enjoy it.

Basically OP you need to stop worrying about upsetting your MIL and just say that you won't be coming because the weekend with 3 kids is too much for you. You could suggest a day out at some point as an alternative.

The trouble is you are fearful of asserting your own boundaries, but your boundaries are perfectly reasonable, so assert them!

You've gone along with this against your better judgement for FIVE years. It's time to prioritise yourselves!! Book a holiday for the two of you so there's no possibility of being pressured into going!!

Funny but true 😄

🤣🤣🤣

I think DGM wants to maintain a relationship between you and DP and his nephews as she realises they will be the only ones left to look after you when you get old.

Your choice, go or not.

This!!!!

I think you just need to rip the Band-Aid off and be honest with her and just tell her that it really isn’t your thing and you’d rather give it a miss this year and hopefully that will set a precedence for future years. I don’t see why you should suck it up and go if you really don’t want to go. If there’s one thing I’ve learnt in life, it’s not to be a people pleaser as it just creates resentment and you end up doing so many things that you really don’t want to do, yes, occasionally it’s necessary but not for something like this I don’t feel.

After 5 years I wouldn't be doing this anymore sorry. This is one of those things you nip in the bud or continue to build resentment inside. And as she said the MIL will just make adjustments and push and push and push to try to make it work.

Ideally her husband needs to speak to his mother and stop this once and for all but unfortunately since he is a wimp she will have to take a stand. MIL will be upset but she will get over it, and if she's doesn't too bad. I wouldn't continue to do something I don't like for years just so she doesn't get upset.

Most people would certainly not live being cooped up in a 4 bed accommodation with other people's kids who wind up their dog.
Sounds horrendous

good god it's not christmas!

The free holiday is a Christmas present from her MIL to them, and she arranges a date during the year they can all make and books it for all of them, all OP has to do is turn up or not turn up.

in laws always know by the way.. I told my sister in the end to stop making up last minute excuses for her ex 'D'H.. normally 'stomach ache' or 'headache', I told her if he doesn't want to come to family stuff he doesn't have to, he's a grown man so why do you make up excuses for him?? In fairness we preferred it when he didn't come anyway because he was a right prick, I just hated the fact my sis felt like she had lie for him.. ugh, in the end she used to say, "he's not coming because he's being an arse". Fair enough.

Come on you must be able to think of an excuse. It’s Christmas, can’t you say friends or your side of the family has a really important Christmas party you have to go to? Your work is doing a Christmas event? There’s something up with the dog? It doesn’t even have to be that believable, it’s just an excuse so everyone gets what they want without any feelings being hurt. I do this with in laws all the time and it seems to work, though I’m not sure they always believe me.

I would go, one night only overnight Saturday Sunday, arrive as late as possible on Saturday and then, oh dear, work, leave asap on Sunday. Hopefully most time there you’ll be asleep anyway.

Here's your answer. How many more Christmases is she going to have? If she's getting on in years, not many. Please don't take her to this stressfull get together and please don't leave her with someone else.

Exactly. I don't believe anyone in their right mind would enjoy being stuck in one static caravan, with a clutch of in-laws, and their annoying children, who insist on winding up your dog. AND you have to pay for the privilege and use up your precious annual leave.

At 27 this would have been my idea of hell! (Tbf, I still wouldn't do it now at 44 with two kids of my own). Bless you for going along with it for 5 years. The trouble is, you've set the precedent that you DO go, so mil will probably assume there's some major issue if you decline now. However, if you frame it honestly then hopefully she'll see that you have been very supportive and family-centric all these years. You are allowed to have preferences and boundaries. I'd speak to her privately asap: tell her how anxious you are about upsetting her and explain that these trips have never really been your cup of tea. You've gone previously because you appreciate her gesture, understand how much she enjoys getting everyone together, and you've wanted to please her. But it just really isn't your cup of tea and you've decided not to go this year. It doesn't mean you'll never go again but you won't be going this year.

Probably best to just speak for yourself, unless you’ve been appointed the spokesperson for most people and I missed the announcement.

You might be surprised to learn that for many people, choosing how to spend their precious free time as they see fit is far more important to them than the value of a ‘free trip’ to some godforsaken holiday park surrounded by other people’s children.

Wow…just wow! No, most people would NOT love it and be grateful! I have eleven grandchildren, but I most certainly wouldn’t be organising my four offspring’s holidays every year and expect them to be “grateful”. Give your head a wobble!

What I don't understand is why you are not able to do your own thing I.e without kids etc

I couldn’t think of anything worse! Six adults and three kids all in the lodge? No thanks. Tell them now before arrangements are made.

It sounds absolutely ghastly and really, you need to put a stop to it before you become so resentful that you blow up on the holiday and really cause upset. At the moment, you can still get out of it. I know, if it were me, that I'd get to a point where I would explode and say much more than was necessary.