As others have pointed out, the legal waters of letting him live rent free are fraught with problems. Do not let him live rent free on paper, even if you don't enforce your lease to the fullest. Make sure he's on a binding lease with a clear time period (I'd go with the standard year to year). Make sure he's giving you a few bucks here and there, and keep records of it. He's family, but keep this transactional.

The reasons for this are twofold: for one thing it gives you some leverage to remove him if he doesn't shape up, but it also holds him accountable in the near term - he can't just sleep it off for a couple of years and hope everything works. It will actually be healthier for him.
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Even if it were, you wouldn't wanna listen to it!

Would you buy the house in this scenario?

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