Is it permissible to inform the police in a country that does not rule by what Allah has revealed, and if I do not inform, will I be sinful if this person steals from others, knowing that informing may lead to harm to the thief's family?

Allah has legislated punishments and penalties that rectify the condition of His servants, and theft is one of the most heinous crimes. The punishment for a thief is the cutting off of their hand. No one is held accountable for the crime of another unless they were partners in that crime. Therefore, it is not permissible to report innocent relatives of a thief if the police are unjust and punish the innocent for the crime of others. However, it is permissible to report an accomplice and a conspirator.