Should I return the money to my brother and my sister's husband after I lost it in a project due to my trust in the contractor's honesty?

The Mudaraba capital is a trust in the hands of the Mudarib (agent), and he is not liable for it except in cases of transgression or negligence. Therefore, guaranteeing the Mudaraba capital is not valid unless there is transgression or negligence on the part of the Mudarib, because making a trust guaranteed by a condition is invalid as it contradicts the essence of the contract.

However, guaranteeing the Mudaraba capital in the event of the Mudarib's transgression or negligence is valid, because it then becomes his liability.

The question remains whether "this man's guarantee" means guarantee in its jurisprudential sense, or merely seeking assurance from him. These matters are subject to the judiciary or its equivalent to decide upon.