Lyric discussion by tommut 

My take is this is a man who is no longer with a girl. But he sees her sister at a party, and she is so like his ex-flame in many ways that he is drawn to her, but he knows it is just the fond memories attracting him, and not the sister herself. And for both of them to give in to the impulse would be hurting the one person they both love - the ex-girlfriend.

An error occured.