While uncommon, Onee-sama can be used to address someone who is like an eledery sister figure, regardless of age.
Much like you can call your co-worker who has worked longer than you at a company "senpai", even if they're younger than you.
In this case, you can infer Miku has been in the business longer than Luka, so it's not odd to imagine Luka looks up to Miku as a superior entity (corroborated by the fact she adds the "sama" suffix which indicates a lot of respect). Even if Miku is younger than Luka.
I know it's uncommon and most fanmade works depict Luka as an elderly sister figure and the general rule is you call someone older "onee-sama" rather than the opposite, but the way this author depicted the relationship isn't inherently wrong.
Though I understand why some people feel like it's weird, but I can't really rewrite what the author wrote so~

last edited at Jun 4, 2015 12:41PM

Awsl