I married the Dragon King instead of my sister

The story

Past-life princess Seraphina forced dragonslayer Cassian to wed her; he loved her sister and died begging never to see her; she burned vowing not to cling. Reborn at her wedding, she yields him and weds Dragon King Vhaeros. He planned her death, but her defiance and tending wounded soldiers melt him. His chest mark matches the young dragon she saved 12 years ago. Destined, they save each other.