Secret Marriage Swap, The War God’s Late Remorse

Someone pulled Perseus out of danger when he was a child, and he grew up sure it had been Leah. The gratitude turned into something else as he got older. By then he was engaged to Cassandra, a Titaness, and Leah was Cassandra's stepsister. Five years into the engagement he was in love with Leah and willing to let Cassandra die if that was what kept Leah safe. Cassandra finds this out by accident. She agrees without argument to a swap of the marriages and takes Leah's place as the bride of Abel, the cursed son of Hades. On the wedding day Perseus works out that the bride has been changed, and none of it makes him glad. What he had taken for love was gratitude, and he had aimed it at the wrong person, because Cassandra was the one who saved him. Over the five years he had fallen for Cassandra without noticing. She is already Abel's wife when he understands it.