How would you split the cost of this trip?

Same price per person

If it was the agreed gift for a parent from the other parent, sibling 1 and sibling 2, then of course each gift giver should pay the same. If there's a massive difference in family size, it might not be a sensible gift to choose, though.

The problem is that OP is thinking of this as shared costs for a family holiday, which should be split per room, rather that a gift given equally from 2 siblings, where the cost should be split 50:50 (or 2 siblings and a parent, with the cost split in thirds).

Quite. I think these group things work best when people are approaching it generously. If I could afford to pay for my other parent's place I hope I'd try to do that rather than insisting that they pay for themselves, much like if I were to buy one parent a night in a hotel, I would cover the cost of the whole room not half of it. But affordability plays a significant role. And it's one thing to offer, but quite another for someone else to assume you will stump up more than you feel is fair.

I think people are getting hung up on this being a gift, when it’s actually a surprise holiday. Take the gift element out, each household pays for the rooms they need - parents x 1 room, single child x 1 room, coupled up child + teen x 2 rooms. In effect, a 4 way split.

The holiday is the birthday celebration, not the gift.

I disagree. If OP and her sibling were buying an expensive present, e.g. jewellery, it would make perfect sense for them to pay 50/50 for the gift.

If they are renting a house for a holiday, the fact that OP's sibling has a teenage child means that they need an extra bedroom, which makes it more expensive. It seems unfair for the single sibling with no kids to contribute to that, particularly if her married sibling has two incomes, her own and her DH's, while the single sibling has one income.

Well then don't buy a joint gift from siblings you can't afford to pay an equal share of.

Presumably, based on this, you wouldn’t suggest paying for this as a “gift” though

It’s not.

If we chose to spend £2400 on a holiday for my parents birthday, there is no way it would be fair to ask my brother to pay £800 of it while I rock up with my tribe and he brings himself.

No, they're paying for 50% of a gift from themselves and their sibling to their parent. Its that simple.

I think it was a poor choice of gift given OP's attitude.

Families often have the least disposable income though - my siblings with kids literally can't afford a holiday - they usually stay with friends or housesit.

So a lot depends on family circumstances and how mich spare money you all have. If money is tight then it sounds like stressful idea!

Doing it this way is hugely unfair on the single child with no children.

They’re paying for a family that isn’t theirs, who will use far more space and resources.

They’ve also only got one income to pay that out of.

Asking my brother to pay for 1/3 of something like this (I have two siblings) would involve asking him to pay for a share of 2 adults and 4 children that aren’t his, as a minimum. If we invited my sisters stepchildren there’d be 2 adults and 7 kids. All of which aren’t his financial ‘burden.’

He likely wouldn’t be able to pay it and therefore wouldn’t come.

Everyone paying for their own people means nobody ends up fronting up costs for spouses and kids that aren’t theirs.

I think everyone is wasting their time. OP clearly just wants validation that she shouldn’t have to pay for others attending the holiday, and won’t see it any other way 🤷‍♀️

But you know in advance how many people are going and it is a gift albeit not a physical one as you want to describe it. Entirely different from a family holiday. You can't expect a spouse or grandchild to pay an equal share of a gift
If you expect the gift to be from you and your sibling then either split 50/50 or just you your sibling and your mother go

But the gift is the holiday. It costs what the holiday that you arranged costs.

If you wanted a cheaper gift, then maybe you should have just arranged it to be you, your sibling and your one parent. Or chosen domewhete cheaper, or maybe not a holiday at all.

In any case, the full cost of the holiday is the gift, and it should be equally shared by the gift givers.

If it wasn't a gift, just a shared holiday, I'd expect the cost to be split by room.

But it is a gift, therefore the full cost of the gift should be split equally by the gift givers (of which sibling's spouse is not one).

You’re thinking of it as a family holiday of which one adult is partially exempt from the cost (although by having the other parent pay you’re costing the birthday parent) and not a gift.

So this is where paying per bedroom comes in, if you have to get a four bedroom place instead of 3, the couple and teen pay for two of the bedrooms, which brings the cost down for everyone else, others are only paying for the one bedroom as that is all they need.. it doesn't matter how many bedrooms there are if there are people in there paying for them and all you are doing is paying for your own, you work out at planning stage if you can afford that or not and accept or decline accordingly.

And It's not just random people turning up, you know what your family is comprised of, who is coming and the accommodation should be worked out at planning stage (as would the cost split normally be).

Have you had a chance to speak to your parent and sibling about how they want to split it yet?

A trip away is no different in principle from any other birthday party. The person who is throwing the party pays for the party, and gets to choose who is invited. If the cost is unmanageable, opt for a more modest celebration.

Think of it terms of a different gift.

You and your sibling decide to get one of your parents a £400 scarf for their birthday.

What would a fair split of the cost be?

a)
You pay £100
sibling pays £200
and parent pays £100

or b)
you and your sibling split the cost of something you two have decided to do?

If you don’t think of it like this, then it’s not a gift.

we would split an air b and b 3 way equally, we do this for Center Parcs. It’s an accommodation cost not a per person cost.

This is one of those where there’s no perfect answer.

As other posters have pointed it out splitting it 4 adults means that the couple are paying a higher proportion of the present aspect of the payment. However you could also argue they’re getting their teen for free. The Spouse of the sibling may also feel like they’re coming along out of familial obligation rather than really wanting to be there, so may not want to pay for the privilege.

Split by room? Well then you need to know the layout of the place? Are some rooms bigger? En-suites? Does teen get an inferior room?

Are the siblings similar in age and on different life paths or is one just graduated and on a starter salary whilst the other is 10+ years older and has had much more time to establish themselves?

Will anyone be getting parent any supplementary gift? Who knows?

I think the difference in cost also matters. Is it £50 or £200 difference depending on how it is split?

Then you can also throw in how are food costs going to be split? If there’s a big shop will it be per family? Per person? What about meals out?

I think if you aren’t going on a trip with people where you all have a generous mindset in the first place all these things can get quite fraught.

Overall OP only you know the nuances of all of this. I think a good natured discussion needs to take placed between the siblings and parent to come to a split you’re all happy with and where no one feels resentment. This will be different in every family and every situation. But going forward I would advise agreeing the split of costs before booking anything.

In our family

Mum and dad would pay nothing as sharing mums birthday room.

Then it would be paid based on rooms unless the difference in day a 3 bed and 4 bed was tiny then we would just 50/50 it. Or rather the adult children of the parent would.

Though again say the teen bunked in with mum and dad or slept on the sofa. 50/50 between the adult children again.

The in-laws would be token gifts because I anit paying say £100/£200/£300 for my inlaw parent birthday gift 😅

Good Grief! Because obviously there would be no need for this thread if everyone was in agreement. And "joint present from the children of the birthday person" implies an equal split between the children of the birthday person.