Kara Zor-El being older than Kal-El is nothing new, as in the comics, Kara was already a teenager when Krypton exploded, while Kal was a newborn. However, through unusual circumstances (it all depends on which story you read/watch), by the time Kara landed on Earth, Kal was already an adult and become Superman. So it’s an interesting movie for the Supergirl movie to follow along with Kara at a time when her cousin isn’t even alive. That being said, what isn’t clear is where Supergirl would be set? Is it following Kara while she’s still on Krypton, or is the movie giving a Superman-less version of Kara’s origin story on Earth? The latter would deviate from the source material significantly, but then again, we did see in Man of Steel that one of Kryptonian pods in the Fortress of Solitude, so at least there’s some groundwork already laid for Kara to have awoken on our world decades before people would hear the name Superman.