I've taken the occasional friend of dcs' away, and would take any money from you in those circumstances, but as, every time this question gets asked on here, the key thing is every family is different and every circumstance is different.
The 'expectations' should have been discussed before anyone agreed to anything.
When we go on holiday we don't really spend anything during the day - we tend to spend time on the beach or if too cold, we'll go walking somewhere. Some families however spend money to be entertained day after day. So, if your dd came with us, we wouldn't be spending anymore, but if she went with the other family then there would be extra costs wherever they went.
We like to eat out when self catering on holiday, so, on those nights, the cost would be higher with an extra person, so that can add up, but it the family cook for themselves and eat in, then they wouldn't noticeably increase.
Some families - when their dc gets invited - are actually really grateful as it means their child gets to go away when they wouldn't otherwise, but for other families, it can be really difficult to find an extra £100 - £200 to give the 'host' family that they hadn't budgeted for, and, if they had, wouldn't have spent all of it on just one of the dc, whilst siblings didn't get anything.
So this is another "it depends on what the circumstances are" question.
However, I wouldn't give much to the 13 yr old, as she isn't going to have the confidence to say "I'll pay for this meal out" or whatever, is she ?