How would you split holiday cost?

Your updates reveal the money is not tight, it's the husband who is a tight prick when it comes to spending money on his disabled wife. Tell him the cost is 800 and treat your sis

I couldnt agree more.

He thinks he can tell all his mates he "treated the missus to a holiday with her sister for her birthday" when the truth is he dropped just enough to cover dinner one night and the OP paid the rest.

Either he pays a significant amount so he can claim bragging rights or he actually puts some effort in to get her a decent present (which he almost certainly wont bother doing). If the OP plays this right she could get some serious money for her sister from the tight selfish , but crucially...LAZY, bastard.

FFS does no one actually bother to read and comprehend anymore.

THE SISTER IS NOT PAYING!!

Her selfish husband who says "money is tight" when the sister wants to do something regularly goes on holiday without her and OFFERED TO PAY TOWARDS IT as the sisters birthday present.....probably because it meant no effort on his part but moving on..... Previously to this the OP invited her with no expectation of reimbursement.

£500 would make you one cf!! It has only cost an extra £130 fir her to come.She will think you only invited her to save yourself £370!

Did her husband know about it before you booked, and offer to put something towards it? If so, you should have agreed on his contribution beforehand.

I'd just work out how much extra it's cost to have your sister added to the trip, and ask him to cover that (or half of that if the idea was for you to treat your sister to a holiday).

I would tell him that between his holidays on his own he can buy your sister a thoughtful present.
I think he wants to get away dipping in 100 pounds and them tell her that he paid for her share and look like a prince.
I may be completely wrong, but I get some fishy smell about his offer to pay for something that is already book, it wasn't his present, he did no planning at all and may be even partly/completely paid.

People saying that she is ripping her sister off, no she isnt. She offered to take her sister on holiday, which is actually what the useless selfish pig of a BIL should be doing but doesnt. What he does is go away with his mates but then say "Oh money is tight" when it is anything for his wife. I suspect he only offered to pay towards it because the OP covering it, when he should have but didnt, shamed him.

So, he wants to pay towards? Good. Start with half and allow him to haggle you down to a third. As I originally said, I would use that to spend on another break for the two of you on his dollar but other PP have made a good point that he may be financially abusing her so she maybe could do with it as a secret fund. So take the money he is offering that he should be spending on his wife as well as himself and discuss with her the best thing to do with it.

And use that week to talk to her and help her make her situation better, because her husband is a piece of shit.

If it's HIS birthday present to her then of course he can bloody well hand over £500!

Other use it's YOUR birthday present to her.

What a tight and selfish git he is.

This^ the child's place is not in reality free, it's a commercial trick.
And then do a trip/tour while there or go shopping with her before the holiday and buy something together .
Dont leave it to BIL to treat her , it seems their set up is a bit unfair.

If I was desperate for a holiday and someone told me they had one booked and could add me on for £130, I would offer to pay a fair share of the total. Because I can't get a holiday for £130, can I? I'm only getting the chance to go because they are going, so why wouldn't I offer to help spread the cost of it.

BIL sounds like a selfish prick who is financially abusing his wife.

Given the updates I’d say 600, and about time he shelled out for his wife instead of just for himself.

Obviously not that tight if her husband can often afford to holiday without her.

She is a full extra person sharing accommodation that would otherwise be only used by you and your son.

On that basis I would tell your brother in law that the cost of her holiday is £940.

This!!
I'd pay for the holiday, as you more or less would be anyway and tell her hubby to give her that as spending money to treat herself

Oops, so it is. Then the hotel room split into thirds, auntie paying 1/3rd.

I'd do this. A three way split would sound fair to most reasonable people so it shouldn't be an issue asking him to pay £600.

If it was a villa you were planning on renting for you and your son, and there was an unused room available, then I would ask them to only cover the slight price increase. But they're sharing your room. They're reducing your space by half, so they should pay half.

if its him coughing up and its not going to cause budget problems for your sister, I'd nail him for the 500 and then either cover your actual extra cost and use the extra for you and your sis to have a truly fabulous time, or splurge the lot on you both on hols. If home circs are really bad you might like to slip her some of the cash to start an LTB fund.

Except the husband isn't coming.

I would ask for a lot more than five hundred then keep some of it back to treat you and your sister to a weekend away at a later date if he is tight when it comes to letting his disabled wife have a holiday while he swans off on his holidays so frequently!

Fact is, you're sharing a room with them if they come. Percentage wise, if you were paying £1800 (ish) for the whole holiday, and they were only paying £100 towards it, they should be entitled to 5.5% of the space. It wouldn't be feasible as there's two of them (him and your sister) and two of you (you and your son). They'll obviously take up half of the space in the room, so they should pay 50% of the room cost. I wouldn't factor in flight cost because your seat allocation and space on the plane is restricted regardless, and I couldn't be bothered being tight about the maths on that.
I would work out what they should pay based on what the hotel room costs and divide it by 2. Ask them that, as they're taking up half the space in your room. That would be fair.

I'd be speaking to my sister not plucking a cost from thin air after it's booked!
And for the record if I was going to pay almost that anyway I'd be happy to pay for my sister.
If you both agree to take money from him then do it and treat her with it if he keeps her short. I'm sure she will help you with your son too.
If she's disabled contact the hotel and hopefully they can accommodate you in a good room that's suitable. Enjoy your sister. I'd love a holiday with mine.

That would be acceptable but pocketing £500 for reimbursement of £130 is just greedy.

It really is. I would either ask for the very slight difference between the 2 or as that's only £130, not ask at all.

Basically for £130 more you've got a holiday for 3 people. Seems a bargain. Asking for £500 is really off.

£500 is fine! It’s still less than a third of the total price so I don’t see why she should pay less.