Buying partial ownership in a house?

[ddubois]

I can't even find any references to this concept on google, except for some timeshare-esque arrangements. I live with my in-laws, in a house they paid for (in full), and the current arrangment is that we pay all the monthly bills, property taxes, etc, but otherwise pay no rent. It's a very sweet deal for us, but they like it too becuase they get grandkids around 24/7.

Anyway, they want to borrow money to travel and stuff. Rather than take out an equity line of credit/loan they want to borrow from us, with the idea "they'll pay us back when they sell, plus interest". My wife has a really bad history when it comes to lending money to various family members, and my policy is "Don't lend to F&F what you can't write off". Well, I can't write off $20K.

So my idea was, rather than a loan and all it's problems, we get some legal document that says we are buying X% of the house for $Y, with some stipulation that when it's sold we get X% of the proceeds. I figure this is a good investment anyway with real-estate being real-estate and this market contniually going up, so it's win-win.

Good idea? Bad idea? Rife with all the same problem that lending money would have?

PS: Here's a picture of some hot chicks: