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?
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.
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.