Thanks for the chapter, KappaBeast.
Isn't that... Normal??
Like how if a brother or father judge their Daughter boyfriends and Mother judge son girlfriend vice versa
What you said is normal, but that's not what's happening here or whenever the concept is included in a story. What's happening here (as with all other instances of this situation) is a third party coming into the picture with the intent to split the two people in a relationship because he or she fervently hates the one she isn't connected to, not just evaluate that person's fitness for the person the third party is connected to. The third party comes into the picture with the aforementioned intent because he or she has already passed judgement--the third party hates the would-be romantic partner because the person
is the romantic partner of the person he/she is connected to.
Yona's sister has no real purpose in the story aside from being a living speed bump for Yona's romance and to appeal to pedophiles. The very fact that Yona looks the way she does--the long hair, soft eyes, large breasts, and a good outfit--means that you will definitely see someone like Yona's sister jump into the story as opposition.
I already said this today about another,
much darker manga: there's pedobait in every manga--there's just no barrier to entry for them.
The thing--Yona's "sister"--doesn't even look female at all. :|
That's supposed to be so appealing to the male audience that it must be included in every manga, somehow.