I would suggest that you buy this place and rent to them, or vice versa. Co-ownership with in-laws sounds fraught with peril. Fences, lawn care, gardening, landscaping, driveways, garages - no way in hell I'd want to involve more people in these decisions, especially ones that might try to sway my wife's opinion.
If there are affordability issues for the buyer, maybe the renter could pay a year's worth of rent up front.
Just my 2 cents. I own a duplex and my in laws live up top. It works pretty well.
I imagine your situation might work out ok in a dense urban area with very little shared amenities/space.