His brother, Numitor, was the King of Alba Longa. Amulius overthrew him and took the throne. Amulius forced Rhea Silvia, Numitor's daughter, to become a Vestal Virgin, a devotee of Vesta, so that she would never bear any sons that might overthrow him. She violated her oaths of chastity by laying with the god Mars, resulting in the birth of Romulus and Remus.
Amulius threw Rhea Silvia and her sons into the river Tiber. The river-god, Tiberinus found them and gave them to a she-wolf to suckle, while he married Rhea Silvia. Romulus and Remus went on to found Rome and overthrow Amulius, reinstating Numitor as King of Alba Longa.
Livy I, 3-5.