Also in first ch it clearly mentioned that mc parents were newlyweds and moved city and there next door family was MLs. As Mc parents have no one to help like relatives there MLs helped alot eachother as growing mc used to stay alot at mls house so it's natural to call them hyung it's similar to calling oppa be female peers to male peers like oppa means brother but women or girls still use oppa to romantic counter parts to hyung as in word is not relation by blood it's more like Hierarchy or age difference word to show respect I think

are they actually cousins or is this a translation error??