How to find out someone's password on the iphone?

I want to go inside my sisters iphone 4 to find out something.well about a month ago I went on this dating website and found this guy I really liked and I didn't even add him as a friend. My sister did! She got jealous or something and won't even tell me his name, email, skype etc.
How do i get inside her iphone?

You have to know the password or you can try formatting (restoring) her iPhone via iTunes which might get her to hate you for the rest of your lives: P so there's no way, unless you trick her to spell it out.

I don't know. Suit your self