According to the opinion that David's Kidushin of Meirav was valid, how could David marry her sister?
Radak (22, citing Sanhedrin 19b): He married Michal after her sister died. 1
Malbim: His Kidushin of Meirav was valid, for he was Mekadesh her with [Sha'ul's] Hana'ah that David pardons the money that Sha'ul owes to him.