Machiavellian Bluff

The hatred that Robert arouses in Mary against Kate helps him gain her trust. Mary plots against Kate and, using Coronation Street wittiness, delivers a mastermind plan to destroy Kate.
Mary, pretending to be Kate's friend, lowers her guard, which exposes Kate to her treachery and betrayal.
Mary and Robert decide that Kate offended them and committed a wrongdoing for which she deserves to be punished.
They stage a simple wrongdoing that the mob could relate to: "Kate throwing Robert's sister out of the house".
They use this "sin" to justify their actions to themselves but especially to blind the public; keeping Kate low, weak and poor by accusing her of the wrongdoing and humiliating her, helps them to become strong, powerful and rich.