You shouldn't fall in love with your step-sister.
Phil's mother just remarried, and he now has a step-sister that is his age.
Phil wants to be close to his new step-sister.
Phil makes an effort to get to know his step-sister and be a true brother to her.
Phil takes his step-sister out on romantic dates and soon starts to feel attracted to her.
Phil is able to develop a strong familial bond with his step-sister.
Phil is separated from his step-sister when their parents become aware of his feelings.