I don't believe that the Female Lead was the illegitimate child of the Male Lead's father (let's call him Father Shen). So there was never any doubt about the leads being half siblings. They were never blood related. My idea is based on the flashback when Father Shen brought Female Lead to his home as a little girl. He told his wife that the FL's mother was dead, and her father was re-marrying and didn't want her any more. Mother Shen immediately accused Father Shen of sleeping with Female Lead's mom, but Father Shen said they never slept together, and he could do a DNA Test to prove there was no blood relationship between him and FL. I'm sure they had the test done (otherwise Mother Shen would NEVER have accepted the little girl into the house), but I think Mother Shen was just insanely jealous of anyone Father Shen liked (and he clearly liked FL's mom), so in her mind an affair MUST have happened, and the FL MUST have been their illegitimate child, and thus FL MUST have been responsible for all the evils in the world, etc etc. None of it was true. There was evidence none of it was true. But that doesn't matter to someone whose mind is a little unbalanced. Edited for spelling