This picture illustrates how even though there are five sisters, Lux is the focal point of the whole film

Lux Lisbon is the focal point of the film and is a symbol of what coming of age in a small America town can be. She is trying to navigate her life from childhood to adulthood though she never fully reaches that point in her life. This film is showing how different everyone's childhood truly is and how it might look like something from the outside it is never as it seems on the inside. The boys that are narrating this story are never identified as individuals, they are shown as voyeurs into the life of this family and that is used to show us that idea of we are all on the outside of someone else's life. Lux, is evolving into a young woman and she is figuring out her sexuality and she is realizing the revolutionary ideas of sexual freedom which is why this is a coming of age film about her more than the other sisters. She is figuring out life and trying to find her place in this world. 

Smoking Lux Lisbon is an example of trying to be an adult 

Lux is trying to adapt to the environment and the social norms that are expected of her, though once her sister passes she realizes that life is shorts and while she had spent time flirting and teasing she hadn't accepted how she was growing into a young woman yet. Once she does however she doesn't know how to be responsible or how to make educated decisions. Lux is an extremely impulsive girl and doesn't stop to consider the consequences of her actions. She is diving headfirst into adulthood without much space in between that and childhood. I don't think she has any idea of how to be in the middle.