TLDR Older sister gets blackmailed by her little sister's ex BF that has videos of her having sex with him. Older sister let him user her body as collateral to keep it secret or else he'll spread it on the internet and a her new BF whom she really loves. By the end the older sister told the EX that she's done with this, he replied if she really forgot the deal but the older sister said she couldn't endure the lie anymorbe and he can do whatever he wants with the videos , older sister told the truth to her little sister and both came to terms with each other. Later we see the Ex approach the new BF and sends him the videos, showing he never was going to commit to his part of the deal. Too bad for the Ex that the bf liked the videos but kept going out with her little sister. His only leverage is gone and new BF got both videos and a loyal GF (she never cheated on him ).
Wrong, it's two alternative endings. One where she gives in and continues the relationship with the blackmailer, other where she defies him and blackmailer goes after the younger sister again