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How was Uthman ibn Affan son in law to Prophet Muhammad (SAW)?
“Uthman was married to Ruqayyah, and upon her death, married Umm Kulthum. Both his wives having been elder daughters of Muhammad and Khadija earned him the honorific title Dhū al-Nurayn (“The Possessor of Two Lights”).”
and Allah Ta’ala Knows Best.