Is the Nikah Still Valid?

Someone wrote: “A man committed immorality with his wife’s sister. Is his nikah still valid?”

(Hadhrat Thanvi responded as follows:) “You ask about the nikah while not asking anything about the evil act which is a major sin. You do not ask about the compensation for this evil.”

If he had asked about compensation for this evil act and about the nikah, then his query would be valid and answerable. If he had written to another institution, the answer would have been that the nikah is valid. The evil act would be brushed aside as insignificant.

(Malfoothaat, Part 8)