Can you fall in love with a person you've known your whole life?

My friend tells me that you can't fall in love with a childhood friend you known your whole life and possibly grew up with, because of the 'westermarck effect'. I do believe that for the majority of humans, this holds true, but isn't an ingrained fact. Its just something that tends to happen for those raised together, But I'm certain that their are exceptions to it throughout the history of mankind. I would just like to hear peoples opinions on this, and if you agree/disagree about that.