This one is pretty easy, but it took me a long time to realize it. You should never lend money to family. I know this seems counter intuitive to what you think the roll of your family should be.
We are all raised to believe that a family should be a loving and supportive unit. I dont disagree with that assumption and in perfect world that is exactly as it should be.
The world isn't perfect though and having someone in your family owe you a debt or owing someone else in your family a financial debt can place a strain on the relationship.
When one party doesn't pay off the other party as quickly as they think they should it can create a large amount of distrust and animosity. It just isn't a great idea. Help them out any other way you can, but dont lend them money.