What is the ruling on an unmarried young man exchanging sexual messages via mobile phone with his sister?

The relationship between a brother and sister is encompassed in Islamic law by what preserves the religion and the continuity of affection. A brother is a mahram to his sister; he is permitted to be alone with her, travel with her, and see from her what is impermissible for others. The mahram must adhere to proper conduct in speech, gaze, and jest, so it is impermissible for him to utter anything that offends modesty or violates Allah's command. Exchanging phrases that describe or encourage sexual relations between non-spouses is among the gravest abominations, as it incites immorality and draws near to zina (fornication/adultery). Allah Almighty says: "And do not approach unlawful sexual intercourse," (17:32) and "and speak to people good words," (2:83) and "And tell My servants to say that which is best." (17:53) Such messages contradict modesty, as the Prophet (peace be upon him) said: "Modesty is part of faith," and "If you have no shame, then do as you please." We advise the brother and sister to fear Allah, abandon this transgression, repent for it, and resolve not to return to it.