They are already getting a deal that other families are not and she needs to be reminded of this. It is not your responsibility to subsidize them further. If she wants to use the two weeks she needs to work it out with the ex, lol, as if that is likely to happen but she is using his week.

At the same time I understand why she thinks she can do it but group fairness is the best excuse to use as to why not.

At the same time if they had worked it out themselves she could have had the two weeks with them each saying one of the weeks was their's and the week you were off was neutral and then they would have exchanged the money accordingly. But since he booked your week first, she is miffed and thinks she is owed well too bad, no you do not owe her the extra week off.