How to read someone's messages on a iphone if it has a lock?

I just found out my stupid b***h of a cousin has been going through my phone reading all my messages.plus she's been very sly about taking the p**s too. I talk to this girl a lot on my phone and i found out my cousin has been going through my phone after she slyly used her name to call me and then she goes 'oh sorry did i call you *** i meant *** ' she said that while grinning at me. I want to get her back by going through her messages and find out some dirt about her.either that or i want to get some form of revenge on her. I know it sounds very immature but i'm just so pissed off right now!

The easiest way is to simply ask for permission, barring that go through birthdays, anniversaries, address, etc.

If the iphone is locked, you can't read any message.