I don't understand why is a issue that they're 'family'.
1. They're not blood relatives.
2. In her first life, they had a strained relationship that was bad, with very little to no communication at all.
3. In this life, even when they have a good and caring relationship, only spent 2 years living together before he sent them away, and the first half of it was as bad as the her first life.
4. Korean females uses 'oppa' not only for their brothers, they used too to their boyfriends and men older than they, so that she call him brother is not that a big deal.
Now, in a unrelated note: the guard is cute 😍