Diamond Valley by Margaret Way
1986

How could she bargain for love? Carradine men only marry women of their own social standing. No one understood this better than Brett, an orphan raised with the Carradine clan on their Diamond Valley homestead in the Australian outback. But when King Carradine's death made Brett an unexpected heiress, the alluring young woman became irresistible marriage material -- especially, Brett discovered, to Jay, King's successor. And even though Brett had loved Jay all of her life, she wanted more than money, or even passion, to be the bond between them.