It's wrong to date a friend's sister behind their back.
Hannah has a crush on her friend Renee's twin sister Carla and wants to date her.
Hannah doesn't want to hurt Renee.
Hannah and Carla go to Renee and tell her about their feelings for each other and ask for her blessing.
Hannah and Carla start to date secretly and do not tell Renee what is going on.
Renee is surprised but understands and gives them her blessing.
Renee feels hurt and betrayed when she finds out and her relationships with Hannah and Carla both suffer.