Consider this panel:
What do ビ and シッ represent? I'm conjecturing ビ is the mimesis for the raised finger, and シッ is the mimesis for the scarf as she turns around.
It is not ビ and シッ but a single word ビシッ. It is a 擬態語 signifying firmness of an action. From びしっと in Daijirin:
[2] 厳しいさま。ぴしゃりと。
びしっと断る (my translation: reject flatly)
In your example, both ビシッ and the raised finger have the effect of showing that the character in the panel is stating her opinion firmly as if it were an objective fact.