Like others have said, you are being the doormat. It's your families company - YOU have the power here. Do NOT clean up the mess. Leave it for your boss to find. (take pictures too) Assert yourself. When he tells you to clean it up, tell him you are very sorry, but that is not in your job description. Make him go to your father to try to get you to clean it up. (assuming you have a good relationship with him) Your boss cannot take any action against you without your father's approval.
This will never stop until you put an end to it.
This will never stop until you put an end to it.