Charlie never really liked his sister, and she never really liked him. They just don't get along, and that's fine. She lives her life, he lives his and at school, they pretend they don't know each other.
Their parents even gave up on trying to turn them into friends.
But then, Marge starts having girls in the house... who like being naked. Maybe his sister isn't that bad.
In the process, however, he will learn how to respect women, and how to be a better man.
How did they become a naturist or what is their relation to naturism?
Charlie doesn't like naturism, but he does like that his sister brings girls who are nude in the house.
He is mostly harmless and Julie doesn't even realize that he likes to talk to them because they are nude next to him.
He isn't even a voyeur, he is just immature and has normal teenage boy curiosity.