When Keisha and Kara Cornelius were growing up in the dusty West Texas town of Denver City, all Kara wanted was to be with the sister who was four years her senior.

For many years, Keisha didn’t understand the depth of Kara’s desire. She’d go out to ride her bicycle with her friends, and Kara — who still hadn’t learned how to ride — would run as fast as her feet could propel her in a futile bid to catch up.