Is a man's love for his wife's sister forbidden (haram), and what is its punishment and expiation if it is?

It is not permissible to be attached to the love of a girl whom one is not permitted to marry, such as the sister of one's wife while her sister is still married to him. Therefore, he must قطع (cut off) this thinking and busy himself with the remembrance of Allah, acts of obedience, and what is greater, such as calling to Allah and uplifting the Ummah. If this love leads to a forbidden glance, an obscene word, or a sinful tryst, then it is absolutely forbidden.