Wait!

He's not. He's trying to hurt Athy because he hates Claude.

Yea but didn’t they mention that Athy and Janet are cousins but they think they’re sisters?

Janet thinks she's *Claude's* father, and therefore thinks Athy is her sister. But Athy and Janet's bio father know the truth. Janet's bio dad DEFINATELY knows Athy is Claude's daughter, which is why he's been trying to kill her/replace her with Janet all this time. Janet was conceived using dark magic and things she touches sometimes become cursed, and so her being around Claude would kill him.

Daughter*

Exactly so Janet is the brothers daughter! But it looks like the brother doesn’t know! I think that’s why he tried to harm her otherwise if he knew she’s his he’ll never do that!