No she's not.
Just
A really loving sister
No, she's very loyal to her Ancia. No cliched evil sister/heroine here.
Spoiler:
She's pretty badass too, actually. Stands up to her father for ancia, and becomes a knight.
SHE DID?
AAAAAAAAAAAAAAAAAAAAAAAA OKAAAY, I'M LOVING HER ALREADY (I saw some spoilers of her so ye)
Thank god to at leasr have ONE ONE sister who is actually badass and not annoying
PLS TELL ME DIANA'S NOT THE VILLAINESS
I JUST WANT A MANHWA THAT THE "VILLAINESS/EXTRA" AND "HEROINE" HAS A PRECIOUS RELATIONSHIP