I am a guy and I had them too, especially about my 4-5 years younger sister from my cousin. Don't know why I've been attracted to them, but maybe they were so interesting cause they are "forbidden" by church and family.
Now, I think that they have connection with my unresolved Oedipus complex and weak intrafamilial and parental boundaries and parenthood - my parents always kept sexuality as a taboo, I was psychologically castrated during my puberty by my mother. I remember one day it happened - I opened my brain for sexual stuff, and everybody else of my friends were talking about sexual jokes like it was normal....
I suggest you read about Electra complex and try to put basic psychosexual developmental things in its place.
Like other said, its only a fantasy.