A Mortal Sin — Margaret Tanner

As the world teeters on the brink of World War 2, Paul Ashfield travels to Australia in search of the mother who deserted him. He meets Daphne Clarke, and after nights of passionate love-making, they decide to marry, but Paul discovers a shocking family secret. He and Daphne share the same mother. Devastated, he quickly departs the scene. In Singapore, they meet again, and Daphne tells him she is not his sister. They marry just prior to Singapore being invaded by the Japanese. In the chaotic aftermath, each believe the other has died during the bombing. When they finally see each other again, it is in an English church, where Paul is about to wed another woman