To un-RSVP to this wedding?

Meh, I wouldn't go. Sounds like too much of a hassle. Spend the £500 on a weekend away en famille.

You probably shouldn’t go as you are working yourself up into a state of righteous indignation and you might kick off at their wedding.

I think you’re overreacting personally. It sounds lovely. You keep going on and on about the money. If you can’t afford it don’t go. My view is as you get older family weddings become few and far between they are a lovely way to see extended family. Think you would be cutting off your nose to spite your face if you boycott in a huff.

If you don’t want to go, don’t go. It’s a long way and lots of days holiday, if I was you. Your shoes i wouldn’t bother. If your husband really wants to go can he go in his own?

So you're saying that they have allocated an on-site room to 50-odd couples but you are not one of them?

Just decline belatedly.
Tell them you've crunched the numbers and you can't do it.
Send them a card and a token gift

3 days off work = loss of time off which costs you in terms of being available for your DCs when they're off school and having to pay/lose money to cover it
Travel costs
Accommodation costs, including extortionate hotel room for a couple of night, meals out, etc, instead of the discounted 'wedding accommodation'
Childcare issues

Just decline.

Or send your husband with the proviso he finds an extra bed in the 'wedding accommodation' which likely exists...

Sounds like they didn't offer accomodation at the venue to those with DC? If they want a totally childfree wedding I get why they'd keep venue childfree.
I don't get child free weddings, but that'd be my guess.

Is it possible they could've drawn names out of a hat for the remaining rooms after the immediate family were assigned? Trying to save arguments but somehow causing more drama?

Go or don’t go. Your choice. 🙂

If you want to foster the relationships: will they be in your area at Christmas/New Year? Would it make any sense for you to host a little informal gathering at home, to "celebrate" the marriage with snacks and wine, in the festive season. Maybe look at the photos from the wedding? Perhaps include others who declined or weren't invited?

That way your husband would get to see his relatives before long, and you would participate graciously in an acknowledgment of the wedding, along with whoever else in driving distance makes sense. Cost you 100 quid instead of three days annual leave, a small fortune and all the faff and travel.

If you toss out that idea when you are un-RSVP-ing, it might take the sting out. Not that I think you should worry about their reaction to your declining; it's a perfectly sensible choice.

Perfect. Polite and to the point.

I get both sides of this, and I think you should support your DH in going if he wants to,

BUT personally when I’ve found myself in these situations I’ve felt much better in hindsight when I’ve said no to being ok with being on the B-list. Honestly - fuck that. People know what they’re doing in these situations. They really do. As long as you draw your boundaries with a smile, and stay polite, then yes it’s fine to basically say no I’m not ok with what this is saying. And I think you should.

For me it’s actually more about your DD too. Bless her, not long started in school and she’s going to be going in over the break, to a school club? And you away for three days? I wouldn’t have been ok with that at that point - she’ll need her downtime, starting school is tiring and emotionally draining and I just wouldn’t want her first half term break to be spent like that 😢

And then there’s the money which would also really sting and I would not be apologising for feeling like that!

I’d say no. I’d send a nice message saying sorry but DD has had a wobbly couple of weeks adjusting to school, we’ve been shocked at the cost of accommodation outwith the hotel and hadn’t realised how much of a difference it makes to the costs for us (😈) and so sorry, I/we are going to have to pull out.

Tell your DH that this is what you’ve decided for you, but if he still wants to go then it will be cheaper, better for DD, less hassle and he/you as a family will have ‘stood up’ for yourselves and made your point without being rude - you will have made it clear that you’ll be gracious but no, you aren’t going to move heaven and earth for them (when they clearly haven’t prioritised you).

That’s what I’d do.

I would 100% back out and I’d have no qualms giving the precise reason.

Paying £500+, 3 days off work and all that travel to attend the wedding when you’re a B guest

The three days annual leave alone would make it a hard "no" from me. We have a longtime policy of only attending weddings within easy reach, that won't suck up our entire weekend. And generally then, ceremony only.

I'm sorry that your husband is having his feelings hurt over this; it's really so thoughtless of the B&G.

Agree with others that they probably expected you to decline.

What a faff of a wedding.
I wouldn't hesitate at all to post decline.
It no longer suits.
The 3 days leave would be enough for me, unless it was someone really dear to me.
Ditch it OP and don't give it another thought.

DH has always felt like that from childhood but this is the first time for a while it’s been shown like this

That's natural though when families break up and start over. Was your DH (and sister) raised by his father and stepmother? Or were they with their mother? These cousins are FIL's family so obviously they're going to see FIL's family unit as their family somewhat more than cousins who they saw less of due to a family breakup. Unless you're going to say that DH and his sister actually did grow up within that family unit full-time, it's not unusual for there to be differences in relationships.

I was in fostercare at 11 yo. I know about childhood trauma.
But.... they are not children. Be carefull to see everything in a wrong light.
Your husband is a grown up. If he doesn't want to go because he isn't close because of their childhood it's fine. But don't make it an excuse for bad feelings over a free room.

But OP, it doesn't mean you're not wanted, just perhaps that they're more in tune with the others are at this stage in their lives?

It's a fact of life that we all have preferences - you do too I'm sure - and at your own wedding you'd really want to make sure that the people you felt you had most in common with were there.

It doesn't mean they don't love your DH or value you as a couple. And realistically, if they've gone for Scotland at all they're not prioritising others' time and won't be valuing (or not valuing) the efforts you're making to get there.

I'm assuming that they haven't paid for all 60 rooms and the other rooms have in the meantime been taken by other guests - in which case, you're really talking about them subsidising others by a matter of £120, but not you. You're not staying at the castle because you weren't quick enough off the mark to get one of the other non-subsidised rooms. It's all a bit annoying and frustrating, but it's not really a biggie.

Just don’t go.

I found myself in a similar situation last year.
Although my DH did not feel excluded growing up and had a nice relationship with the bride and the groom, we decided not to attend. The bride and the groom have the full right to organise the wedding the way they want (also a remote location and child-free, the accommodation covered only for selected family members), you also have to full right not to attend for logical reasons you've mentioned. In our case, I didn't like the lack of consideration for the guests, regardless of how Instagrammable the location was.

I would be inclined to say no, that now you've looked into it, accommodation round there is just too expensive to justify it.

By putting it on specific invites only, they have very obviously hand picked the people they want staying in the hotel with them, which you could argue is up to them but to expect everyone else to have to pay £500 when the hand picked people are paying £120 for accommodation is a piss take imo.. they could have at least organised options elsewhere or offered to lay on transport for the people having to stay further away at much more cost.

They obviously aren't bothered if the B list go or not, so for those eyewatering prices I'd decline.

If you don’t go, you won’t to have to give them the money present either.

If you would like to see the elderly relatives, do it in your own time. Maybe take them out for afternoon tea or something memorable for them and you.

That’s such an empathetic and carefully thought out response.