
I mean idk too much about the Korean language but I'm pretty sure he can't just say "stay away from my sister" bc he'd just use noona and noona is a general term for close female friends older than you (male) by a lil bit. Someone correct me if there's actually a way for him to express that sentiment without making it seem like he's dating his sister lol

Noona literal translation is older sister. In Korea they call noona to older female friends but also to their older female siblings. That's why when a guy introduces his sister to someone he will specify that it's his "shin noona"= real sister. So unless the say it like that people won't think it's his sister but an older female friend.
Congratulations on finding the crown prince and the chick that he likes.