How to confront a pathological liar if they won't admit to a lie?

My roommate is a pathological liar. She lies about everything, but I've never actually been able to catch her because she's so good at it. For example today she told me she's going to italy this summer with her sister even though she's told me before she has all brothers. When I called her on this she quickly back tracked and said she really means her cousin, and their just so close that she refers to her as her sister. She mentioned before that she's not close to any of her relatives outside of her immediate family so I said I thought you weren't close to any of your cousins? She looked kind of flustered for a minute and then was like yeah not really she just invited me to go to italy with her. Yet she calls the girl her sister wtf? Why does she lie about the dumbest things and how can I get her to fess up?