In Illinois 5 years if the contract is oral and 10 years if it is written. If you have a written agreement then the 10 years starts from the date of the most recent payment or written promise to pay. If the sum is substantial you should hire an attorney to assist you with the matter.
Some kinds of agreement must be in writing to be enforceable pursuant to the Statute of Frauds. Your post reveals too few details to know for sure. So, see a local lawyer with all petinent information to determine if the agreement you had with your brother-in-law is enforceable or not