That was such a stupid movie (spoilers)
first of all i figured it wasnt his daughter from the beginning. literally the very beginning. i didnt know her exact motive but figured she was prob in love with or obsessed with the dad and wanted to seduce him, but i figured she was lying. but then right when he spilled wine on her shirt, and was basically fondling her boobs. the fact that from the beginning and even as time goes by their son is attracted to her to the point where he cant help himself and kisses her (if they shared actual DNA, and he was told this could potentially be your sister, he would have been able to restrain himself. and he prob wouldnt have been attracted to her. because its not natural to be attracted to people youre actually related to. unless youre some sex crazed maniac. yeah he was a teenager but that shouldnt have been the excuse all the time for him being attracted to his sister and kissing her. he also had a huge crush on a girl in his grade, so sorry no, that also showed me she most def was not telling the truth. but then especially the way he was touching her all the time, and by the time he yelled at the guy the BBQ i was like oh, right, they are both trying to get rid of her so they can be together. but what a dumbass plan. the writers/director really thought this was clever? "Yes ill pretend my mistress is my daughter..... tell my son who will still be living with us it's his sister....tell EVERYONE all the neighbors and everyone i know my mistress is my daughter, and continue living in the house as one little happy family" ?? there are many other ways to make your wife crack (like continuing to do what you were doing, just basically touching ur "daughter" and acting romantic verses fatherly everyday) instead of getting her fired again. Or simply telling her youre leaving her for a random 19 year old girl would have done the trick. But trying to get her to crack by messing with her job....effing stupid because thats your current lifeline you moron.
How bout you just keep flirting with your "daughter" in order to make ur wife go crazy again so she goes back to the hospital --- wait a few months and keep seeing her in secret-- and then introduce her as ur new gf. yes everyone will think you're a scumbag and hate you but eventually people, especially your own son would have caught you kissing her anyway and youd be an even bigger PERVY sick scumbag to everyone. So yeah it's just as evil but at least it's not a effing stupid as hell plan. Whoever came up with the idea for this movie, and thought it was clever, is a total moron.