You're boned. Write it off as a loss and lesson learned and move on.
You can either negotiate an agreement that has no legal force behind it (take what you can get), consider it a loss, or sue. There aren't really any other options here, and if you want a good relationship with your brother in the future, either sticking with the current agreement (or a similar one) or forgetting about it are your only choices.
I never "lend" anyone money that I intend on getting back, because most of the time you're simply not
getting paid back, and it's just charity.