Buying a house -- Help! -- Family members still living in one unit and want to stay on as renters

[John Christy]

I am buying a house and closing next week. The house is a multifamily (two-flat) but a single family lived in it. The owner (husband and wife) are selling and moving across the country but they'd like their son and his family (daughter attends school nearby) to be able to stay rent the top unit. That would be fine with me, except:

They'd like to pay $800 a month and comp rents in the area are $1200.
I am closing next week and neither the sellers, their attorney, nor their agent have made any attempt to get in touch with me and arrange for the son to be on a lease.

I think they're just 'hoping it will be cool' and 'we can work it' out after/during closing, but I'm wondering if they should be out of there at closing, because if I come in and ask them to sign an $1,200/per month year-long lease, they're going to be pissed and say they don't want that and say they'll move out, possibly trashing the place in the meantime, and if I sign him on a month to month for $800 and give him a month to move out, I'm saddling myself with difficulty if he trashes the place or doesn't move out, etc.

I just got in touch with my agent who will definitely get in touch with the seller's agent and ask about this, but before deciding what I'm going to tell the sellers when they do contact me, I wanted advice. If I said 'no rent, no lease, no nothing,' and the son's whole family had to move out next week, and go who-knows-where, they'd come break in, trash the place, turn the whole neighborhood against me later anyway. Seems like there's no way to do this safely and benevolently. And if they sign a lease with their son now, they could make it for $500 a month and I'd have to honor it . . .