How would you split the cost of this trip?

Splitting it four ways may seem fair but it's not really as the couple with the child are paying twice as a family for the non paying parent.

I'd personally do it by bedroom as the fairest way:
So single person: one bedroom
couple with teen: two bedrooms
paying parent: one bedroom

BUT if you insist on splitting it into 4 paying adults. You should first split it into FIVE, each paying adult pay their own portion then split the non paying person's share (one fifth) into 3.

Have you ever said who's idea it was to do this trip?

It really does have bearing on what is a 'fair' way to split the cost of it.

Was it the siblings or was it the other parent? Or did you all get together and say let's do this all together (which in itself infers a more equal split of the costs)?

It does make a difference.

But I'm pretty sure your sibling agreed to split the costs with you, not pay twice as much as you. If my sibling and I decided to split the cost of a present for our parent, the expectation would be a 50:50 split, because we're both equally DPs child and we're both equally gifting it.

You want someone, who is not a child of the birthday person, to pay for a big chunk of the present that their children have decided to give them.

Your IL wouldn't have expected to pay for the gift that you and your sibling are giving your parent, because people don't generally go round buying their PILs expensive holidays for their birthdays.

I think on out case the siblings would pay equally for accommodation (if one siblings family has 2 rooms then they would pay a bit more. They would offer. I wouldnt necessarily ask). Then food etc. Would be divided per person and the siblings would pay for their family so the one with partner and teen would pay for 3. We would pay for both parents. If non bfay parent would insist jn paying, we wiuld just divide it between siblings afterwards.

The point is, you’ve arranged to buy your parent a gift split with your sibling.

You then want the other parent to pay 25%
Your sibling to pay 50%
and you to pay 25%

It would be split by four in my family and everyone I know. Just like Christmas, one spouse wouldn't say that the other had to pick up the cost of visiting, because there's no adult blood relation to them, present.

When it came to food, again split by the adults, with individual alcohol carried or if buying litre bottles/mixers, split costs and the parents pay for what their children want.

It’s not other people’s money, it’s our money. I don’t have a separate “it’s my mums birthday next week” pot?

For example, we’re going for a family birthday meal next week and the money will come from our shared funds anyway, but I’d think my partner incredibly rude if he just didn’t turn up.

Living that separately and picking apart parents birthdays to that level would wind me up 😂.

Because its a gift from siblings to parent to pay for the holiday. So they pay for the holiday, otherwise its not a gift from them.
If they dont want that gift to include a room for partner and child, then don't include them.

Its not OK to spend other people's money.

Whats odd about it? The expectation that you can't ask someone else to subsidise an expensive gift you've chosen to give to your parent?

I might be losing my marbles here, but if I was going on holiday - for any reason - why wouldn’t I pay for going?

When we went for my step dad’s birthday last year we paid both mine and my partners share. I don’t think it would have been appropriate for him to not come, and it’s our money that paid for a break we were both going on.

Surely if you’re with someone long term, there’s an expectation they come on family breaks if invited because they’re part of your family, and it wouldn’t be free for them to come because they’re a paying adult?

You’re clearly the single adult and are miffed that you have to pay a share of your siblings spouse and teen 🤔

Maybe it's not that they "refuse to get the point", more they don't happen to agree with you.

Very simply, I think the full cost of the gift should be split equally by the people who decided on the gift and who are giving the gift. So really 50:50 between the 2 siblings. I think that the other parent may also want to help the kids out and chip in too, which, if parents can easily afford it, the the siblings should graciously accept whatever the parent offers. I dont think an IL should be expected to contribute to an expensive holiday that they probably are only going on for their spouses benefit.

My siblings and myself would just split the cost between us. But we are a close family and would never expect one person to pay more for having an extra child.

Thinking about it, if my brother in law and husband arranged a holiday for us and their parents and his brother expected me and my husband to cover half the cost, I’d definitely stay home with the kids.

Hahaha you make me laugh @GetBendyWithWendy .. you’re being hostile to those who have a different opinion to your thoughts. You are assuming lots about me, none of which is true 🤷‍♀️ I really don’t understand why you’re being so cagey about giving some information 🤷‍♀️

But even so, if you and your sibling choose the gift, you pay for the gift. You dont have the right to spend other people's money by deciding on an expensive gift and expecting someone else to pay for a big chunk of it. An appropriate gift for a PIL is a box of chocolates, not an expensive holiday!

I think the two adult children should split the cost 50:50.

It’s not irrelevant and you’ve been hostile and rude.

But who has arranged it and wants to go?

The spouse might not actually be that keen and be happy to stay home with the kids if it means they don’t occur half the cost.