Honestly, the first thing that came to mind when I saw this was Cinderella. We’ve met Ashley (was that her name?) and Anastasia but we’re missing the third. And they’re going to a ball. Hmmmm……… Don’t think they’re planning on messing with the Elsa and Anna story that much by bringing in a third sibling.
But this sister would hate her step sister…this one loves her sisters. Also, we’ve already met the Ugly Stepsisters in Cinderella Centric episode. @Rainbow suggested for her to be E. Mitchell and Belle’s Mother’s sister, that’s the best headcanon I’ve heard thus far. Because screwing with the Frozen Tale by adding another sister….I just don’t agree with.
"If you go as far as you can see...you will then see enough to go even further." - Finn Balor