Brianna was born from a werewolf-her father and a vampire-her mother. Both of her parents fell in love with each other but when both of their leaders found out about their relationship they were told not to see each other, but her parents found a way for them to still see each other. It so happen that them seeing each other became frequent that Brianna mother became pregnant with her, her mother tried to hide her growing stomach but when it got to big to hide she stayed inside.
All of that change when she misses a clan meeting causing the clan leader to come and look for her which resulted in the leader finding out she's pregnant.
Brianna mother was given until after birth to kill her child because they already knew who the father was, but her mother didn't want to kill her very first child. So she ran away with Brianna father but little did they know that after a few years they would be attack by their own people and that her mother was gonna be killed during the attack.
After her mother was killed her father and her kept running for years until he decided that it was time for them to stop running and get a house and for Brianna to start a life, going to school and making friends which seems impossible to Brianna. That's when they move to Forks, Washington.
(Rosalie is not in this story)