While Kara was older than Kal when Krypton died, by the time she arrived on Earth, Kal was already an adult and protecting humanity as Superman, resulting in him taking his now-younger cousin under his wing.
Besides the 1984 Supergirl movie and the Supergirl TV series, the Girl of Steel’s appearances outside the comics include Superman: The Animated Series, Justice League Unlimited, Smallville and Superman/Batman Apocalypse.
Given how many DCEU movies are in development, we’ll have to wait and see whether Supergirl actually moves forward past the scripting stage.
However, assuming that Supergirl does become a higher priority project, then it’ll be comfortable alongside The Flash, as both heroes are currently being represented in CW’s Arrowverse right now.