How can stolen money be returned to its owners without their knowledge, and is it permissible to conceal the matter from the wronged father to save face, knowing that the other partner has passed away?

You must return what you stole from your father and your uncle to each of them, or to the heirs of your deceased uncle. If you do not know the exact amount stolen, then make your best effort to return an amount that you are certain is no less than what you took. You are not required to inform them of the theft; rather, it is sufficient to return the money by any means that does not cause you harm or embarrassment, such as sending the money anonymously, with a note explaining that it is from someone who has repented to Allah. You must strive in repentance and seeking forgiveness.