French language Q&A Forum

I understood that the par or devant were essentially the same - both mean to pass by something.  I answered passer devant in this questions just to use a different way of expressing the same meaning.  It was, however, said to be an error and passer par was the correct answer.  Please explain.  Thanks.

Another problem I have here is with the explanations in the lesson. It says that when talking about SOMETHING SPECIFIC, you should use elle/il. But then there's this example: Qui est Sylvie ? - C'est ma sœur. 

I would think that here we are talking about a very specific Sylvie, and not just some Sylvie. Then why do we say c'est instead of elle?