Translation: We must stop them from doing what we intend to do to the hero. Or it is fine if we do it.
Also what they are doing is basically a self fulfilling prophesy. If they tore them apart the maou will resent the kingdom for taking away her sister and in the process hurt/kill their father. It is better this way. Maou will learn love and would not even think of making war. Hero would also do her best to stop humans doing the same.