Is the mutual love between a young man and a young woman—who had not met nor spoken except through a third party (the sister), and then married years later—a sin requiring expiation?

Love that is not caused by a person is not blameworthy, unless it is followed by a forbidden act. However, love that arises from repeated looking, unlawful mixing, or correspondence, makes its owner sinful. Correspondence between the sexes is not permissible due to the arousal of temptation and the frequent occurrence of evil therein, as it can lead to many corruptions and circumvent the Lawgiver's prohibition of seclusion.

Shaykh Ibn Jibreen said: It is not permissible for any person to correspond with a strange woman due to the temptation involved, and the correspondent may think there is no temptation, but Satan tempts both of them.

A woman is forbidden from speaking softly with a man who is not lawful to her, "So do not speak in a soft voice, lest he in whose heart is disease should covet, but speak with appropriate speech" (Quran 33:32). And it is forbidden for a man to derive pleasure from the speech or voice of a strange woman.

Making one's sister an intermediary for conveying messages is another evil. What is obligatory is to repent to Allah and increase in righteous deeds.