Should the employer be informed about theft committed by relatives against him?

What your relatives are doing by stealing is unlawful (haram), and you must denounce their actions and remind them of the gravity of transgressing against a Muslim's property, especially if the transgression is committed by someone entrusted with the money. If they do not desist, inform the employer, because enjoining good and forbidding evil is obligatory. You must inform the owner of the money about the theft, otherwise you would be assisting the thief in sin, for Allah Almighty says: {And cooperate in righteousness and piety, but do not cooperate in sin and aggression.} Your relatives must return the previously stolen items to their owner.