In the novel's ending, the mystery is finally resolved. The identity of 'A' is revealed, and the girls are able to put an end to the constant torment they've been through. They also learn a great deal about themselves and their friendships along the way.
Well, they're sisters. Alison was sort of the 'dominant' one in their relationship. Courtney had her own issues and was in and out of places like mental institutions. Alison, on the other hand, was the 'queen bee' in their town. Their relationship was full of secrets and lies, which was a big part of what made the 'Pretty Little Liars' story so interesting.
It was Sasha Pieterse. She did a great job of differentiating between Alison and her twin, even though they were played by the same person. She was able to show different personalities and mannerisms for each character, which was really impressive.
The existence of Alison's twin added a major twist to the story. It explained some of the strange appearances and disappearances that had been confusing the other characters. It also brought in new layers of mystery and danger as the twin had her own motives and secrets.
One major event was when Courtney tried to take Alison's place in their social circle. Also, there were events related to their family secrets. Their relationship was a key part of the mystery in the show.
There are fanfictions where Alison's cousin has a secret past related to the 'A' mystery in Pretty Little Liars. In these stories, the cousin's actions can either help or hinder the Liars as they try to figure out who 'A' is. For example, the cousin might be unknowingly working for 'A' at first but then has a change of heart.
One common theme is love and redemption. Their relationship in the show had so many ups and downs, and in fanfiction, often they are exploring how to truly love each other and redeem themselves for past mistakes. Another theme is mystery, as they still have to deal with the secrets of Rosewood even in their relationship.
Spencer is also very popular. Her intelligence and competitiveness can be the center of many fanfictions. Writers can write about her academic struggles, her relationship with her family, and how she uses her smarts to try and figure out the identity of 'A'.
One idea could be exploring a different backstory for one of the main characters. For example, what if Aria had a secret relationship with someone before she moved back to Rosewood. This could add a whole new layer of mystery and drama to the story.
In the end of the Pretty Little Liar novel, the girls triumph over 'A'. The revelation of 'A's identity gives them a sense of relief and they can then focus on rebuilding their lives and mending their friendships.