Is it permissible to caress a half-sister who is wearing provocative clothing?

You must immediately repent of the crime of intimately touching your wife's sister by abandoning the act, feeling deep remorse, and resolving never to return to it. She, too, must repent for enabling you to touch her. Thank Allah that you did not commit zina (fornication/adultery). If you are still living together and her husband is absent, you must leave the house or she must leave, to prevent the recurrence of such immorality.