I don't think she can appreciate the gravity of the situation because she has absolutely no idea how detrimental the situation is, her mother certainly didn't tell her anything. And her mother really has no idea how detrimental the situation is either and she knows more. We the viewers know it's a zombie apocalypse and the epic scale of poo about to go down. These people have yet to figure out what exactly is going on. I do agree with you though, the teen angst shtick gets old really quick including her "I'm a part of this" brother.
Also, I do believe the brother faked the seizure to prevent his sister from leaving. Grabbed a mouth full of soup and put on an acting job. That and later on he made the comment to his mother that he stopped her from leaving.
So, I just re-watched the episode. I think the barber shop family is harboring, or at least the wife is, an infected person. Maybe she wanted them in after the husbands refusal as a means to an end. I also enjoyed the ole switcheroo with the radio comment "This changes everything. This is a catastrophe of biblical proportions. We've just lost the best pocket passer in the NFL".
If you recall the principal was eaves dropping on class I think in the episode last week. This week while eaves dropping he heard sounds and a lot of disheveling going on. My guess is he went to investigate the sounds and that is where he was bitten. I could be wrong I may watch it again later for clarification.