I have a plot question...

I just caught a screening of this at SXSW and loved it, though I do have one question. What did Celia need convincing about, that she had to go back to Andrew's house with him? Initially I thought she was under the impression that Paul was her father (due to Paul's "you're not the first person to think that about their parents" comment during the rain storm, in response to Celia's question of "what did you ever see in my mom?"). The whole ordeal with her finding the picture also gave me this idea. So, I thought that Andrew came along and told her that Paul wasn't really her father, but rather her half-brother. She didn't fully believe him, and went back to the house to see the will so she could be convinced.

That's the impression I'd gotten from all of the events. However, during the last scene of the movie, Paul and Celia are talking for the last time (this is before Andrew ever comes along to tell her whatever it was that he told her that caused her to go back to his house in search of proof). During their conversation, Celia asks something to the effect of, "so you knew that your father and my mother..." to which Paul replies, "yes, but I never knew about you."

So my question is this: when and how did Celia find out that Paul's father was actually her father? And part two: if she already knew about and believed that Paul's father was her father, what did Andrew come along to tell her (and subsequently take her back to his house to prove by showing her the will)?

Thanks in advance.

-JP