1. Yes, written partnership agreement. Two problems: 1. you can't foresee every problem so you will have to work problems out--This can be an issue since it's family 2. It's family so even if it is written you won't follow it.
2. Yes, property managers will do the work--cost about 10%+ of the rent, depending on the area.
3. Better option. It is a promissory note on the property. If you don't pay they can and should foreclose on the property (makes Christmas uncomfortable). Not screwing your sister. Parents are being paid for the level of risk they are assuming--less risk, less reward. Are you happy with 8%?--if you put 50% down a bank would be better.
4. I would avoid family deals unless you don't have other options. Someone will always feel they did more work, etc. You will be mad at your parents, your wife will be mad at you, your sister/parents will be mad at you/wife. Hard to sue business partners. Really hard to sue family. It'll be great until the first down market and your parents need cash to pay for X or you need cash for the kids braces.
I would do separate deals. Help your parents find something and improve it. Work for free. Let them make the final choice though.