What would you do to force your brother in law to return the 15,000 dollars he borrowed from you ?

So, my husband told me that his brother needed to borrow 15,000 dollars from us in order to invest in some property in China. Well, he claimed that his brother's mistress recently won the lottery there after doing some "research" and winning 300 million yuan (FYI, I'm really not that stupid, I'm sure that she lied when she said she won all that money. HOW THE ******* HELL CAN YOU RESEARCH AND WIN THE LOTTERY???) Anyways, I'm for sure that both my brother in law and his mistress are both in it to scam me, but my husband's either too stupid to see that or he's also in on it too. I just need a way to get my money back from that jackass cuz my daughter needs to go to college soon!!!! Anyone got some tips?? (on how to get my 15,000 dollars back of course) my brother in law claimed that he'd return it to me 2 months ago, but he hasn't. I'm giving him one more week to hand over the money but he most likely wont. Any advice ?