My argument is that I’m not subsidising her adult working son while he’s living under our roof we’ve worked years to pay for with no intention of moving out.
The reality is someone else living there costs very little as council tax, water and elect, broadband etc are largely fixed costs as is the mortgage
YOu are neither subsidising him nor charging him the small increase in cost you actually have as a result of him being there
IMHO the fact you dont want to “subsidise a partners child” is the real issue here not how much you can force him to pay because you basically dont want him thre
No he puts his foot down kicks out the daughter and then he remains happily married for ever after.
Not much you can do when its someone child as their love for them may well be greater [ or at least equal to] their love for you