Well now that he actually did stick his cobra inside her I'd say the fight will be between na eul and da heul cuz the big sis doesn't seem to even think of him like that, maybe because of her kid but who knows what could happen.
Given my exp with Korean manga I'd say soon some stupid guy obsessed with one of the girls will make trouble, and we can't leave the assistant girl out as well cuz she gives off the feel of having her sights on MC.