Based on a book written by my beloved V.C. Andrews, Christopher and Cathy fall in love when their mom and grandmother lock them and their younger siblings in an attic. Can you blame them? There was nothing else to do but take baths together and fall in love! Later on in the book series, after they marry and have kids (!!!), Chris and Cathy find out that their mom and dad were actually half-uncle and niece, as well as being half-siblings (look, if you want to understand this better, I suggest you read the book series, which is AMAZING). So, you know, incestuous love runs in the family.