Your sister must be a special kind of person to have opened her home to your son.

It does sound like he could wind up in your basement indefinitely. I don't see how even a written contract could prevent that. How do you evict him, if he hangs around too long doing nothing constructive? Do you say: "Time's up . . . hand in your key . . . there's the door . . . now hit the road." Then - what if he simply declines to leave?" Do you call the cops?

I see you've got a real dillemma.