Guillermo del Toro's latest movie, Crimson Peak, covers a lot of ground when it comes to genre, but in short it's a gothic romantic horror — although not the kind of horror movie you would expect. In fact, there are a lot of unexpected things about this movie — namely, Crimson Peak's incest plot. As you might expect, spoilers ahead!

There might be something between siblings, as it turns out.
Incest was certainly not a topic I expected in a movie like this, but surprisingly, the practice was commonplace in gothic novels. In an interview with io9, Hiddleston said, "The thing about gothic romance is, there’s always a sexuality at play, and incest has been something that’s been hinted at by other novels in the genre."
That's because it is a part of life. Flowers in the Attic is cited, of course.
It is interesting to see how different films tackle the subject: foreign and art house films, for instance, tend to handle it delicately, with insight and intrigue. The French film Ma Mere, starring Isabelle Huppert, tackles the classic Oedipal complex, depicting the amoral relationship between a mother and her son after the death of his father. Similarly, Academy Award winners Julianne Moore and Eddie Redmayne played a real-life mother and son, Barbara and Anthony Baekeland, in 2007's Savage Grace. The two allegedly had an incestuous relationship, which ended with the murder of Barbara at the hands of her son.
Murder is so... blah, apparently. He goes on to cite "Oldboy."
Then there's Shame; Michael Fassbender and Carey Mulligan play brother and sister in the 2011 film, and although there isn't any blatant sexual interaction between the two, their relationship alludes to incest based on something that happened to them as children. It apparently made him a sex addict and her unable to have a relationship. Again, there's an element of punishment.
We can't depict consanguinamory in a positive light, now can we? I mean, sure there are real-life examples out there, but, you know, let's not do movies about that, right?
With more mainstream Hollywood movies, incest is often treated like a punchline. This is most evident in 2001's Say It Isn't So where Gilly (Chris Klein) finds out that he might be sleeping with his biological sister, Jo (Heather Graham). The movie is essentially a 95-minute joke that could very well be told by your drunk uncle during Thanksgiving dinner.
Yes. And then he goes on to cite "Back to the Future," "17 Again," and "The Empire Strike Back."

I sure wish someone would turn these works into a good movie or series of movies. Maybe a television series? Consanguinamory has always been in our literature and so it isn't surprising for it to be in movies.