He should sell the rental property if he doesn't wish to be a landlord anymore.
Do not go into business with your wife's siblings and that is exactly what you would be doing. Don't go there!
That gut feeling you have..... yeah, the one that led you to post this thread is trying to save your life, your marriage and your finances. Listen to it. Put your foot down, just say no, then duck and cover.
"to duck and cover" - eh...
That is price less... lol
You guys have been helpful; if only after a few comments.
Here is a flash-back to the 50's - via You Tube
http://www.youtube.com/watch?v=jitctNWozxY&search=duck%20and%20cover