The BBC seriously has some of the best documentaries. I just viewed one called Brothers and Sisters in Love, and as the title would suggest, it's about incest. Most of the couples featured live as husband and wife. Some of them even have children together. (And some of those children have severe birth defects.) Initially, while watching, you want to cringe…or ralph. But one couple, who was by far the most "taboo", were able to explain their lust for each other really succinctly. They are a mother and a son, and she was forced to give him up for adoption when she 14. 27 years later they reconnected, and, as they explain it - they had a deep desire, that wasn't necessarily sexual - to be one. They both wanted him back inside her. They are both disguised in the clip above.
Haven't studies shown that our feelings of 'ick' about incest come from living with someone and growing up with them? But when we haven't grown up with our (siblings for example), the natural sibling love doesn't have the added 'ick', and that's why there's a problem with incestuous sibling relationships where they haven't grown up together.
Does that make sense?