hyungnim is the formal way to call an older brother, adding "nim" actually signifies politeness and utter respect towards someone. hyung/hyungnim is technically used between guys but there are (rare) occasions that girls use this, those who doesn't want to come off like cute and adorable and deems calling someone oppa unsuitable for them. I guess jihyo just want to show deep respect towards kookie and that she answered very seriously to him out of nervousness.