So being a lender you will be paid back within ? number of years with a return of ? percent.

You decide what you should get out of the deal and and you two agree on it. Look through the idea and purposal and see if you think he can do what he promises, if not talk it through and see what you or he might be missing or just have opposite views on. He may think he can make so much money by doing thisbut you dont, therefore you may not think he can be as successful as he is predicting.
Above all, remember he's family, and even being family you can say no. It may be hard, or easy if you dont like him(joking), but I may not be a good idea to mix business and family that directly.