When Numitor 's brother Amulius became king by force , he made Rhea Silvia a Vestal Virgin , so she would not have children who could be kings instead of him . But the god Mars seduced her and she had the twins Romulus and Remus . Before their conception , Numitor 's brother Amulius deposed his brother , killed his sons and forced Rhea to become a Vestal Virgin , intending to deprive Numitor of lawful heirs and thus secure his own position ; but Rhea conceived Romulus and Remus by either the god Mars or the demi-god Hercules .