The Little Giant of Aberdeen County, by Tiffany Baker

Truly Plaice was born so large that her mother died giving birth to her. Her sister, Serena Jane, is her exact opposite: small & pretty. When their father dies, each girl goes to a separate home to live with family friends. Truly goes to live with the Dyerson's, a poor, out-of-luck family with a daughter her same age, Amelia, who rarely talks. A few years later, Serena Jane is married to "Bob Bob" Robert Morgan, who comes from a long line of Robert Morgans, all doctors. Bob Bob takes Serena Jane to Buffalo, where he goes to medical school & Serena Jane gives birth to a son, Robert "Bobbie". Meanwhile Truly takes care of the horses & does other work on the Dyerson farm. Serena Jane & Bob Bob move back to town & live in the doctor's house, while Bob Bob starts his new practice. Truly moves in with Bob Bob to take care of him & Bobbie, age 7, when Serena Jane leaves. Bob Bob is ashamed that his wife has left him, so he invents the story of her death, and Truly doesn't know the truth. Truly's friend Marcus returns from Vietnam, with injuries to his hand & leg. He becomes the gardener for Dr. Robert "Bob Bob" Morgan. There is an ongoing idea of romance between Marcus & Truly. Truly discovers the secret shadowbook of Tabitha, the first doctor's wife. She begins mixing herbs to try out remedies, and finds that there are herbs that can heal, but also herbs that can cause death. Bobbie moves out the house at age 18 with a secret of his own, estranged from his father. Soon after, Robert Morgan is diagnosed with cancer, and after his death, Truly learns the truth about her sister, who is still alive, and feels betrayed by Amelia, who knew the truth but didn't say anything. This book was hard to stop reading!