These types of agreements can start wars even in the warmest families. But if you're convinced that your family can weather disagreements about buying, selling, renting and using the property, then build a few precautions into your contract.
First, set down some rules about the percentages of ownership accorded each party and what rights those percentages confer. If you put in 70 percent of the money, does that mean you're entitled to 70 percent of the proceeds from rent? And if the property is sold, do you get 70 percent of the sale price? It stands to reason that you would, but just to be on the safe side, get it in writing. Sometimes people remember numbers differently when money is involved.
Also, are you allowed to sell your share of the property? If so, can you sell it to anyone you want?
In any event, you want to make sure you know your partners in the deal very well, and make sure they're willing to spell out every detail of the rights and privileges accorded with joint ownership.