Should petrol money and accommodation be charged?

Yes, person C should pay their way. Or if person A invited them and offered to cover their expenses, then they should settle this with person B, so that person B is only paying 1/3. It entirely depends on what the original offer entailed, but no, no one should assume they are getting a free holiday.

Yes, they should pay or cover food/entertainment costs to the same value.

I would want to if I were C, would feel awkward otherwise.

Person C should be paying the lions share for food if having free accommodation!

I think it depends on the amount of money involved and everyone's situation. If I was person B, could comfortably afford the holiday, and liked person C, I would be happy to sub them on holiday. I'd view it as an opportunity to do something nice for someone in need. Everyone's different though, and person B should certainly be offered a third of their money back either from person C or person A.

OP, am I right in thinking you're A and you have invited a family member to go with you, B has agreed but you don't want to ask C to contribute? If so, that's not a good way to treat B tbh. Apologies if that's not the case. If you're B, then that's not a nice way for you to be treated!

They should pay the costs. Doesn't matter if the trip was planned way ahead without them. Person B is now finding themselves in a position of having to spend a holiday with someone they didn't plan on hanging out with, potentially spend the vast majority of the trip rehashing their difficult circumstances or whatever. It is a very different holiday from what was originally planned and I would at least hope to get reduction of costs as a compensation for change of plans.

How many bathrooms are there? There's a huge difference in the inconvenience of sharing with one and sharing amongst 3!
How did A 'ask' B and did they do so before C? Were they guilt tripped into as it sounds like it!

C should pay 1/3 of course. It's a different holiday with C there so for this new holiday the costs are different. Unless C is sharing a room with A or B? In which case you might want to divvy things up a little differently (though they should still pay 1/3 of fuel).

Did b also say they were happy to provide C with a free holiday?

B probably assumed C would contribute because that would be normal.

I would say to C that accommodation is already paid for but she can cover (all of) the fuel as her share. Need to know amounts really to know if that's fair but seems the easiest way to do it.

C should pay a third. No one expects to have a holiday and not pay anything. Or they shouldn’t, anyway.

Who is debating? If it’s A and B why doesn't C just pay the 1/6 share to the one who wants it? If it’s C not wanting to pay anything they are a CF. It isn’t hard to make 2 bank transfers or cover the expenses like food/drink/activities on the trip. If C really can’t afford it then it’s up to A and B to decide if they want to cover all costs and if they don't C can stay home.

I got the letters wrong which I’ve gone back to change but apologies if I’ve missed any!

If I was in that situation and I was ok with c to tag along rather than having it forced on me be by b then the way I see it.

a and b were happy to pay half for a two person holiday

c coming costs nothing if there was a room going spare anyway. So i probably wouldn’t want the money towards it as I was already happy with the costs to me before that point.

I would however expect c to pay their share towards the extra expenses so food, fuel and pay their way (or at least not have It cost me anything) if we all went out to eat or do an activity etc.

that all depends on how close I was to A and if C was not known as a cheeky fucker before this point if you know what I mean.

if I knew c was skint and not a piss taker in normal circumstance and I was particularly close friends with A then I might even discuss with A chipping in more between us to cover C’s the food as it prob won’t be too much for 3 instead of two

Where would it end? Person B also decides to add an extra person to share their room and so on…

Is person B happy with the relative coming? It will change the dynamic a lot.

But yes they should pay a fair share, as A & B will have less communal space, longer wait for shower etc.

If A chooses to ‘treat’ relative then they should compensate B for the compromises that will be made by covering petrol or a food shop or whatever.

Just re reading and this makes me laugh incredulously! so paying for accommodation would involve person C handing over cash to person A and B to make it so they have essentially all paid a third each.
You make it sound like this is ridiculous! Actually having to pay for own accommodation?! The horrors! C isn't actually being made to go on holiday!

Of course person C should pay a third.

They're not tagging along, they're having a holiday.

If anything person B now becomes the tag along person so perhaps they should have the free place??

They should all pay a third. Why should the others subsidise that persons holiday.

This,of course . should pay, does B even know them well enough to want to go on holiday with them, never mind fund it!

Person C should pay a third. Especially as you’re staying in a caravan, an extra person in that space makes a big difference.

No one said C was struggling financially. OP says C doesn’t want to be alone right now, so I’m guessing that C actually heavily hinted that they wanted to go too.

You sound like you’re person C?

Yes, it was all paid for and they were happy with the costs split between the two of them when they booked. But their holiday dynamic absolutely will change with you there, whether or not you involve yourself in decisions, so you should be contributing to the costs.

Course they should pay a third, or if they can't, then the person who is magnanimously inviting them should cover their costs ie two-thirds.

If you invite someone on holiday because they are struggling financially, you need to make it clear you expect them to pay as early as possible. You should have told them when you issued the invite.