Shanna by Kathleen E. Woodiwiss. I found a stack of her books that my mother bought in the 70s when she was in college. They were my first introduction to romance novels and I read every single one of her books. Ms. Woodiwiss passed away in 2007 and I know she is missed by many! She will forever be my queen of historical fiction.
In 1931, after their mother's suicide, the young Junan and her sister, Yinan, make a pact never to leave each other. The two girls are inseparable—until Junan enters an arranged marriage and finds herself falling in love with her soldier husband. When the Japanese invade China, Junan and her husband are separated. Unable to follow him, Junan makes the fateful decision to send her sister. Inheritance traces the echo of betrayal through generations and explores the elusive nature of trust.