Kim Perell has a fraternal twin sister with whom she always felt competitive. Now, Perell has boy/girl twins who are 11, and identical boys who are 6. She encourages each to find their own interests, ...

This as-told-to essay is based on a conversation with Kim Perell, author of "Mistakes That Made Me A Millionaire." It has been edited for length and clarity. My family is knee-deep in twins. My twin ...