Amy Preston is the maternal half-sister of Lucy Preston.
In the original timeline, Amy Preston was born in 1990 to Carol Preston and Henry Wallace. She had a degree in sociology and a black belt in karate. (“The Red Scare”) She often tried to get Lucy to leave their mother's place of work in search of better opportunities, but Lucy was reluctant to do so. (“The Lost Generation”)
In the altered timeline, Carolyn and Henry never meet, causing Amy never to be born. (“Pilot”) Lucy was distressed over losing Amy, but later decided to continue on the missions, hoping that some day, she'll be able to go on a mission to save Amy. (“Party at Castle Varlar”)