Harpagon turns to his son Cléante and commands him to receive Mariane kindly, since Harpagon knows that sons don't usually like to have stepmothers, especially when stepmom and stepson are the same age.
Cléante agrees that he isn't happy to be getting Mariane as a stepmother. And he isn't, but not for the reasons Harpagon suspects. Cléante promises to treat Mariane well. You know, because he hearts her.