If you are interested in this because of the supposed taboo ... well relationship that may or may not be ... consumed ... well let's just say that this is a drama rather than a wet dream (or nightmare, depending on where you stand on the issue).
Furthermore as I'm certain others may have explained too: this is not as taboo as you may think or some may have made it to be. First of all the actress was an adult at the time of shooting and supposedly even if she was 17, her future step brother 19 (so no blood relation to begin with - not even a tiny bit) - not something that would be criminal in any sense. And since there is no blood relation there is no other crime here either - if anything was to come of the whole thing. Hypothetically speaking ... still there is more than just a whiff of awkwardness flying around.
Having said that, there is also an interesting subplot with Asia Argento who looks lovely as ever ... and plays another broken character - which also is something to be expected. If you are aware of certain things and don't expect this to be anything this is not (see above, only a little bit of nudity) ... the movie actually delivers.
