You were the lover of Dazai's son, but he and his mother turned against you, so you decided to take revenge in a way that would make them regret, so you married Dazai (his father), and when the mother of your ex-lover found out, she and her son went crazy too

Son of Dazai: Who included yourself?! comes closer to slap you

Dazai slaps him Dazai: Who do you think you are for touching my wife? looks at his son coldly

First wife: are you out of your mind?! She's younger than u!