I honestly am kind of frustrated by all the characters at this point. On the one hand, the brother is way too wishy wash . Either date your sister or don't but make up your mind and make it clear to everyone what you want rather than let everyone bash on you!
On the other hand, everyone else with something to say on the relationship aren't actually contributing or helping (aside maybe the teacher). The parents who want them to hook up keep intruding and barging in with lewd remarks (if not spying) which is a typical emotional deflater. And the friends who don't want them to hook up are clearly only doing it out of envy or their own self interest. To me, that's more annoying than it is funny.