When Brainiac attacked Krypton, he took Kandor with him, while Kara and her parents remained in Argo.
Kara is actually older than Kal-El (Superman); she was a teenager when he was a baby.
She is the cousin of Superman, Lana Lang's foster niece and a member of the Justice League of America.
(For now at least.) Kara-Zor-El, also known as Supergirl and Linda Lang, is a Kryptonian superhero based in Metropolis.
During the journey her rocket was caught in a large kryptonite-encrusted meteor and she arrived on Earth years after her cousin did.
At the climax of the struggle, Supergirl unexpectedly left (with her comrade in tow).
Since she did not consult with Power Girl about the timing of their exit, the two parted on somewhat strained terms.