I was wondering, are the ages of 18-20 generally considered stretching it for a YA short story?
My main character is eighteen at the moment, but her sister is 19, and her boyfriend is twenty.
Is your MC still in high school, or has she graduated and moved on with her life?
Depending on that and on the themes of the story, it might be New Adult or even adult rather than YA. Age is not the only determining factor, though in general, yes, 18 and up would be stretching it, in my opinion.
I'm currently reading The Indigo Spell (Richelle Mead) which is considered YA. The MC is 18 or 19 and the Love Interest is 22 (I think). Mostly though it's probably the themes that categorize it as YA.
It's probably also the fact that the series it's a spin-off from (Vampire Academy) is YA, and there are crossover characters? (Could be wrong, I've never read either series). Same with how The Piper's Son is about adults but it's categorized as YA because it's a sequel to Saving Francesca.