Behind the Revenge.

Ponwalai, Yodthian's sister, is Prakob's mistress. He devoted his money and himself to her, but she already had another man. When Prakob finds out, he dies of a heart attack and leaves lots of his assets to Ponwalai, which results in his son Noppharuj being furious at both her and Yodthian. Noppharuj chases after them to get revenge, and Yodthian lets him do whatever he wants because she feels guilty over her sister's actions.