I don't know if I would say that to her. How about saying instead that you'd like her to sit down with you and do a budget and then agree that no money gets spent unless it's on the budget. When she wants money for a babysitter, she can budget for it, and do without elsewhere, or find the money some other way (second job?) Don't put restrictions on her, but don't give in to her either. You don't have to fund her fun, but you shouldn't be telling her no either. It will only lead to resentment tied to "DH didn't let me do 'X' - he's so mean" This may not work as often aliens don't want to live in reality, it's the ultimate escape that she's partaking in and budgets and money are in the real world, so she doesn't want to deal with that, but it's way better to have the paper/budget tell her no than you do it.
