Stepmothers in the Bible

Wow, as I was reading the Bible, I realized that Sarah may have been the first stepmother in the Bible.  She could not have a baby so she told her husband Abraham to conceive with the servant.  When Abraham's son was born, sarah wound up conceiving and having her own son.  She no longer wanted her stepson in the picture and had Abraham send him out of the community into the woods to starve. What a stepmother!  Was Sarah the first stepmother in the Bible and if so, who was her mother-in-law?