Incest is either biological or emotional, not social. If you marry a person, you just have some social relationship with his family, not any biological. Or even if you have emotional relationship with your in-laws, that doesn't mean anything to you own biological family in any way. I really don't understand why people name any connection 'incest' without any context
Idk about you but it's common in all places. Like even in western countries, it's normal for pair of siblings to marry/date other pair of siblings from non-related family. It's same goes for aisan countries. I don't know from which place you are to find this things confusing ٩(๑❛ᴗ❛๑)۶ or maybe it's difficult for you to name and understand relationships. It happens to many people
I think I am the only one who ship whole ki sisters and whole kki brother ٩(๑❛ᴗ❛๑)۶