Fatal Vow Swap, I Rule the Castle

The Kingdom of Eloria has strict social classes, and nobles hold all power. Viscount Moore has two daughters. The elder sister Lirayn spends half her life working nonstop to lift the fallen noble Vorian to the top of power. Her stepsister Elysia marries Kyran, the heir of Duke Hale, and suffers endless bad treatment. After Kyran's family loses its wealth, she envies her sister's good life and kills Lirayn.