I have made the same reflection. It almost looks like she has to ask Scott for permission before she does something, and he has always taken care of her. To me it looks like big brother/ little sister relationship.
Scott must have approved the nights activities since the first thing Stifler does men he enters Benjamins and Scotts place is to gp up and talk to Scott.
It sometimes look like he is her older brother. He is always (almost) protecting her and telling her what to do and not to do (at least that what it looks like. I can´t understand a word of they say)