What should I do with the stolen item in my possession, now that I have learned the details of its theft from the thief himself, and have become hesitant to return it to its owner for fear of being accused or of the thief's involvement being exposed, and what sin would I incur if I do not return the trust to its owner?

It is permissible for you to conceal your sister's husband's sin, as long as he has repented, based on the Prophet's (peace be upon him) saying: "Whoever conceals a Muslim, Allah will conceal him on the Day of Resurrection." Do not investigate his other thefts; rather, advise him on the obligation to return rights to their owners as a condition for repentance. The stolen item must be returned to its owner by any possible means that does not cause harm or a religious prohibition, such as sending it by mail or hiding it and informing the owner of its location, while clarifying that you are not the thief and that the thief has repented. As for the statement attributed to the Prophet (peace be upon him), "If you knew the unseen, you would choose what has occurred," it has no basis in the authenticated books of Hadith.