What if Anette was the one who had cruel intentions!
It just came out to my mind when i saw the ending, when Annette drove Sebastian's car with a weird smile and somehow not showing any grieve at all. And suddenly all those flashbacks that show 'romantic' memories of Annette and Sebastian together were shown (or in my theory, flashbacks that show moments where Sebastian finally fall into Annette's arms)
What if, Annette was the one who planned to destroy these siblings (and we will never know why, but one thing for sure, she had already known Katherine and Sebastian more than they knew). And she started it by fake writing article in a magazine about how proud she was being that girl who wouldn't have sex outside marriage and made sure that Sebastian would read it, because she knew that Sebastian love all those challenging kind of girls. Besides that, it actually quite confuses me how easy she changed her mind and actually be the one who 'invite' Sebastian to have sex with her. Soooo somehow she managed to catch the siblings's (especially sebastian) attention and have them playing in her game. Sebastian's dead, Katherine's reputation was ruined, and she had Sebastian's jaguar as a bonus. Well, these siblings were no more than her toys. Oh! And it actually explains quite much about the poster (at least for me lol)
Well just a thought of someone who's craving for plot twist