@Byonbyon Although oppa does mean older brother. It's too casual and informal especially in these types of stories (noble talk). It's like saying ni-chan. The original use orabuni (오라버니) instead. If it were Japanese it would be onii-sama. And yes i agree.
@Neramix
You're correct, as you might have noticed with the fancier clothes.
The title of this part in the LN/WN is
Also, here's the title for the other parts... (and her age)
Part 1: Daughter of a Soldier (5 ~ 7)
Part 2:
Part 3:
Part 4:
Part 5:
Spin off / sequel from another...