No the red/pink hair girl is not from both family. They explained it in early chapters that their families always buttheads with each other. I think the sister took that oppurtunity to beat up the brothers in that family
Ian is thinking about him and jaeho how will jaeho’s family accept him since they were rivals. They didn’t have a good past which includes his sister beating jaeho’s sibling
So the redhead gal is not from the family? then why was Ian talking about the fact that his sister had to beat up the other family? still confused