
that doesn't make them directly related in any way, all of the other relationships you named are weird in their nature even with no blood relation, but THIS is such a mental gymnastics. why make things weird, when it's about two completely different people with different upbringing and different parents like omfg. friends dating is also weird to you? they may know each other's families even better and actually grow up together then what

Never said directly, directly would be straight up incest. Also not really mental gymnastics, in layman's terms, it would be a relationship between 2 in-laws, and if the ml and mc got married their own siblings would become in-laws unless you don't expect the mc and ml to get married?
Also I don't think you fully read my comment, I pulled the definition from Wiktionary and in laws was included in that definition so I don't see what the confusion is about

I mean, you are right and wrong at the same time. Yes, relationship between in-laws would be something of a pseudoincest. And no, hyung and hypothetical brother would not be considered in-laws. The only in-law relationship after marriage is between the spouses and their partner's family. Not between family and family as a whole. But that is a general view, in different cultures it might vary
Hawt . and what if the ml has a brother, the brother of the mc and ml can be the side couple ╮( ̄▽ ̄)╭ (guys don't come at me this is just my what if)