Well apart from growing up together in an orphanage I really can't think...o my

Leonheart : "Princess Hilda never told you what happened to your father"

Fry : "She told me enough! She told me you killed him"

Leonheart : "No Frioniel, I am your father"

Fry : "No...that's not true...that's impossible!"

Leonheart : "Search your feelings, you know it to be true"

Fry : "Noooooo! No..."

Well anyway, I'm really stumped on this one. Cousins maybe?