This Supergirl has only been around for 3 years or so. They got rid of the one that was around since Crisis (the one with the insanely confusing backstory because DC at the time didn't want any survivors of Krypton other then superman as they thought it diluted the character). So in a 6 issue arc of superman/batman in 2003 or 04 i think they re-introudced her as the cousin of Superman who surivied. They had Michael Turner draw the arc. It was extremely populer

It's kinda convoluted so here's the wikipedia entry

Issue #8 of the Superman/Batman series originally published in 2004 re-introduced Kara Zor-El into DC continuity. Like the pre-Crisis version, this Kara claims to be the daughter of Superman's uncle Zor-El and aunt Alura. Unlike the traditional Supergirl origin, Kara was born before Superman; she was a teenager when he was a baby. She had been sent in a rocket in suspended animation to look after the infant Kal-El; however, her rocket was caught in the explosion of Krypton, became encased in a kryptonite asteroid, and she arrived on Earth years after Kal-El had grown up and embarked on his career as Superman. Due to this extended period of suspended animation she is "younger" than her cousin, relatively speaking (she is referenced to be about 16, while Superman is portrayed to be about 35+). At the end of "The Supergirl from Krypton" arc, Kara officially introduces herself to many of the heroes of the DC Comics Universe, adopts a Supergirl costume, and accepts the name.

A new Supergirl series, written by Jeph Loeb, began publication in August 2005. The storyline in the first arc of Supergirl depicts a darker, evil version of Kara emerging when Lex Luthor exposes her to Black Kryptonite. The evil Supergirl implies that Kara's family sent her to earth to kill Kal-El as revenge for a family grudge; at the time, Kara herself refuses to believe this. But later flashbacks show that not only was this true, but Kara had been physically altered by her father as a child, before being involved in several murders on Krypton.
So after all that they went for the "She doesn't know who she is and her role in life" situation and that's how you get what you saw in Brave and the Bold.