Commercial property purchase?

My father in law has a proposition for me to buy the siblings out and own 35% of the building. I dont see moving back inside, but I guess I would have FREE outside parking and a 35% investment on this building and lot, located in a prime commercial area. I can afford the 35% buy in, and my exp. for the build.(tax, insur) would get covered from renters. How does this process start? DO i need a purchase agreement drawn up? Its a bit confusing, but the 3 siblings own 2 properties, and my father in law is signing off on the other and the other sibling is signing off on this one. I am not using a loan, I have the money, so it will be paid for. What do I need to do to make this deal legit, and protect myself. Is it as simple as changing the deed, and a small doent stating my % owned? I trust my wifes parents....especially since she will also be listed on the deed. Just need to know what processes we need to take to get this property transfered correctly.