Beyond the Mountain — Nan Asquith

After her parents' divorce Emma Conway had seen little of her father, and now that she was going to Switzerland to visit him and his young second wife Hilde it was really only out of a sense of duty. However, Emma settled down reasonably happily at first, and managed to sustain a friendly relationship with her young stepmother.
Then Emma met Kurt Alther, Hilde's handsome cousin, and everything changed when she fell wildly in love with him.
Only .....did Kurt return her feelings? He paid her a lot of attention, but Emma could not avoid the suspicion that she was only a stopgap, that another woman held a more important place in Kurt's life - and she grew uneasily more certain who that woman was. While all Greg Burnham, who loved her, could do for Emma was to wait until it was time to pick up the pieces.