1. First contact your sister-in-law with the iou, negotiate with him, and ask the other party to fulfill the contract and repay the debt.

2. Secondly, agree on a new repayment method, and you can ask the other party to write a repayment plan.

3. In the end, the negotiation failed and no agreement could be reached. You can bring a lawsuit to the court with sufficient evidence, claiming to exercise your creditor's right and make it repay.