I honestly thought this was a joke when I first read the title. But actually, uh, yes, you made a mistake. And I have actually heard of addicted gamblers calculatedly using the play you have been played with on their friends and family.
What it is, is that gamblers really do intend to win, secretly believe they're going to win, and fully intend to pay you back when they borrow money from you. And sometimes, they borrow money from you, actually do win, and actually do pay you back, at least a little bit. And what does this do? It makes you about fifty times more likely to loan them more money the next time! It's a great investment for them. So sometimes, these people will when they're flush, borrow a little bit, pay you back, which sets you up psychologically, and then later they move in for the bigger amounts.
Remember, they always did, when borrowing the money, sincerely believe they were going to win, did intend to pay you back, yada yada. They can be very charming. And it is so reinforcing when they do pay a little bit. You have the choice of hanging around, and seeing if he pays you a little more later. That'd be the best way to insure you do get paid at least a little.
The wiser thing to do would be to try to get a judgment against this person and get him out of your life, regardless of how charming, related, whatever he is to you. Gamblers have ONE priority. Paying their debts isn't it.