IMO, sit them down and say this: if you wish to use our hot tub, please go look at three apartments, get pricing and come back with the paperwork and we'll sit down and discuss it.
1. They're living dirt cheap!
2. The cost of a 1 bedroom apartment will probably shock them
3. The cost of utilities and deposits will floor them both.
4. Add up those expenses and the cost of insurance, food, etc...
Now.....here's your choices:
1. Start paying $500.00 rent a month (still very cheap) to use the hot tub
OR
2. Continue paying $ 400.00 a month and don't even think about using the hot tub!
Then ask them which they'd like to do. Sometimes it takes tough-love to make some people see the reasons behind it.
My daughter is living in our other house for $50.00 more than our mortage payment on it. I sat her down and explained that she would NEVER find a house to rent at that price ever and that we are doing it because we love her and want her to get a chance to get some cash saved up to purchase her own house. We could have rented that house out for over a $1,000.00 a month but we decided to let our daughter rent it for a few years at 60% that cost. She knows that she's lucky to have such a deal (she's done her shopping on rental properties) and she's also lived in two other apartments in the past.
Don't feel at all bad about it, you asking her to help around the house shouldn't be an issue with them only paying $400.00 a month!! That's dirt cheap.....I can't remember the last time I only paid $400.00 a month for an apartment, it's got to be close to 20 years ago!