This book would have been published in the late 80's-early 90's. The story is set in the US in the 1800's. The heroine is orphaned and comes to live with her cousin's. The male cousin (I believe his name was Cole) and she fall in love, they struggle with their attraction (and guilt) and ultimatly succumb. She gets pregnant and marries a neighbor (evil) to cover up the pregnancy. Her husband and cousin confront each other and she ends up in the way and the baby is lost.

The female cousin (Lizette?) was slim and pretty until she hurt he ankle and became chubby. Her story is parallel to the main characters. She and her brother's friend fall in love despite her chubbiness. I THINK that the title or the cover had an oak or willow tree.