My partners ex has requested he keep paying maintenance for his 18 year old who has decided to stay at home and do a course at college rather than go away to uni. He'd already said he would give the 18 year old a set amount while they were in FE as he did with their sibling but his ex has said that she wants the money - not just the amount agreed, but the amount he had to pay while the child was at school which is considerably more. If she were in financial difficulty is understand, but she is better off financially than us. My partner wants to be fair but this is causing strain on us all. I personally think, at 18, there needs to be some independence and the opportunity for budgeting and earning but I don't want to get in the way of their co-parenting. To add to it all, my partners ex is now off on a ten day holiday with the expectation that the 18 year old spends the whole time with us (not asked, just assumed) and I've said to my partner this isn't OK. We had a much needed child free weekend coming up and I feel like a misery for telling my partner I don't want the 18 year old there for the whole time. Any sage advice from other co-parents out there? Am I being unreasonable for wanting my partner to set firmer boundaries with his ex?