If Michal Was Barren, How Did She Have Sons?

Chapter 21 of the Sefer Shmuel (the Book of Samuel) refers to the five sons of Michal being hanged by the Givonim (Gibeonites.)  This is a problem in and of itself as earlier we learned that Michal was childless as a punishment for her indignity at David’s dancing before the Ark of the Covenant.  Interpreters explain that there were five children of Merav, Michal’s sister, who Michal had raised as if they were her own.