Your brother is taking advantage of you because you let him. There's these things called boundaries. It's your job in life to establish boundaries that help you control your life and keep out the drama, deadbeats, a-holes and whatnot. Control of your life should be your goal not making sure your brother is the only one who benefits from your work which is the likely outcome here.
I once worked for my brother for free because I strongly believed in supporting family and figured we were on the same team. Two things changed my mind: 1.) He complained more about the great free work I provided than he complained to people he paid who did lower quality work. 2.) I caught him paying someone else a huge amount of money to do what he was happy to let me do for free. In other words, he had the money, he just didn't respect me enough to pay me even a token amount. Mind you, I had paid him cash so he could do his work for another relative in the past. And he didn't say, 'no'.
Speaking of point 2... when you give someone time (money) they put it towards what matters to them. Not you. For example, if a client begs you for an extension on payment they will still pay their light bill, maybe even go to the movies or buy a new car. They just aren't paying YOU.
I was a lot younger and more naive back then when I learned about my sibling. Now, I've seen it all and at the end of the day, unless you have a joint bank account with your brother, "keep it moving". Get real clients. If your brother is good at business he can succeed without you. If he's not he will fail and that's on him. I'd give you a different opinion if he'd given you a more reasonable deal but the one you described is a joke. The same kind any con artist off the street would give you.
But, OK, you say, 'answer my question'. To make sure you get paid, you need a contract where the payment is not based on his performance but is just based on the work you did. And gives you the right to audit his books so you can see what he's making. This is all a hassle and you're not going to want to pay a lawyer (worst case) to sort it all out when you're dealing with relatively small sums of money. A good business principle to follow, is not to lend money unless the borrower is credit worthy. You are giving your time (money) to someone who isn't worthy. You are hoping what he's doing will pay off. He probably has zero record of taking the kinds of things you're giving him and turning a profit so there's super high risk here. Your best move is not to do something so risky.