How do i fall out of love?

I am completly in love with my sisters ex boyfriend. And he's in love with me. but i just can't do this to my sister anymore. Its hurting me knowning im doing this to her. I've tried everything. It just won't work. I know its wrong and such. And please don't reply saying im a bad sister etc etc etc. Ireally dont wanna hear it. I just need help falling out of love with thi guy. Please help me.=(