RHEA-SILVIA had a rather nasty uncle called Amalius who wanted to ensure he was next in line for the throne of Alba Longa.
He made his niece worship VESTA so she would be forced to take a vow of chastity. She became a Vestal Virgins and was strictly off-limits for the next thirty years.
However MARS had other ideas and followed her into the woods one day. This resulted in twin boys.