Environmental influence does affect one’s personality in one way or another. The influence can be varied from one individual to another, so different individual might display differences and uniqueness.
Does parental favoritism affects personality? Well, if you look at personality traits according to OCEAN, there might be different spectrum of personality traits. So, some researchers would look at link between one variable to another in a simpler form. One such study is about perceived parental favoritism and its relation on narcissism. What do you think?