Paul's Father

Fantastic movie, the depth and intricacy of the story kept me guessing at every turn but as an avid reader the movie changed many key features. This is ok in my view (the director should have some room for imagination) but I have issue with the director (or writers) new view on Paul's father. They have turned him from the kind, open minded man that he is and turned him into a man who has no issue with having a sexual relationship with his son's girlfriend with results in Celia being Paul's half sister. this just seems to have gone to far

What is your opinion?