I guess you haven’t read Jane Austen books.
By the way, cousins getting married is legal in many European countries like UK, Netherlands, Germany, and in the half of the states in the USA, and in many other countries. Law allows such marriages (though public morals don’t).
Oh, and you definitely shouldn’t read manga Saraba Yoki Hi.
Not that I liked the ending of My secret brother, it felt forced.

Like another comment said I don’t care if it’s cousins because it’s fiction.
So this is my third time reading since this came out years ago. It was a great drama that kept me hooked the whole way. The main problem I have with the story is the time lapse. It feels so lame. Nothing about their situation changed when it came to their family/ friends so they just wasted all those years. We don’t get to see the outcome with Hanmi’s mother or them actually start their relationship/ more emotions from Hanmi/ things changing when they get together etc. Because I want to see more about Hanmi feels and them going out for real for the first time without it being Iike 4 years later or whatever.