Amy Preston is the maternal half-sister of Lucy Preston.
Amy is the daughter of Carolyn Preston and Henry Wallace. According to Lucy, she has a degree in sociology and black belt in karate. Amy spends most of her time taking care of Carol, who has terminal lung cancer and is bed-ridden. When Lucy is denied tenure at their mother's workplace, Amy encourages Lucy to quit and find a better job. Though Lucy is reluctant, Amy is persistent, telling her sister to "make her own future." (“Pilot”)
After Lucy travels back to the Hindenburg and returns home, she finds out that Amy was never born because of the timeline change. She later finds out that Henry and Carol never met, because Henry married the granddaughter of a Hindenburg passenger. (“The Assassination of Abraham Lincoln”) Lucy was distressed over losing her sister, but continued on the time traveling missions on the condition one day, she'd be able to save Amy. (“Party at Castle Varlar”)
Lucy eventually got permission to go back in time and get Amy back, but she was prevented by Rittenhouse and Carol from doing so, because Carol was desperate to stay alive. (“The Red Scare”) Emma Whitmore later told Lucy that Carol had her go back in time, and make sure Amy could never come back, which devastated Lucy. (“The War to End All Wars”)