What is the ruling on someone who secretly took his father's money to trade with it without his father's knowledge or the knowledge of his siblings, and is there a sin upon one who knows about this for keeping silent about it, and is it permissible to work with him or borrow from him?

Your cousin committed a grave sin by betraying his father's trust. He must return what he took of the money. It is not a condition to inform the father of this; rather, restitution, repentance, and seeking forgiveness are sufficient. You must advise him on the obligation to return the money. It is not permissible for you to work with him or borrow from him if the money belongs to his father, because he does not own it. The Prophet, peace and blessings be upon him, said: "The wealth of a Muslim person is not lawful except with his willing consent."