If I remember correctly
Mc’s sister was friends with ml and ml family protected mcs historically ? They’re family’s are friends
MLs fam is the protector of MCs fam, from their old ancestors time, they have this bond & promises. MLs ancestor sworn to protect MCs ancestor even aftr generation up to MC & ML time which is the present. Bcos MCs fam have this curse that they will not reach above 20 or something. MCs 2 older sibling are close with MLs (ML is the oldest, another female, then youngest is Muryeong which is MCs closest friend). That is why ML is overprotective (even the whole fam) of MCs bcos he is the only remaining Seo. To put it simply, they are not relatives but close fam friend, super close.
Can someone explain the whole relation thing like who’s connected to who like I’m saying the two brothers and then the friend and then their relationship like ik there’s the high school one but with the younger brother