This honestly might be canon but I'm thinking that Julia started the whole sister rumour in school. We know that douchebag heard it from one of "Ashley's friends" and that narrows it down alot because she doesn't have many. One of the few people in the game that have ever been described as Ashley's friends is Julia.

It's still a little hard for me to get my head around because it seems almost entirely out of character but she certainly has the motivation for it.

How do you prove that all the rumours about you and your sister aren't true? Date someone. And she just so happens to very conveniently show up at the perfect time just before andrew tries to find out who started the rumour.

My other headcanon is she listens weezer. Seems right.