What is the history of alison and Courtney in pll?

In "Pretty Little Liars," Alison DiLaurentis and Courtney DiLaurentis are twin sisters with a complex and tumultuous history. Alison, the manipulative and popular leader of the group, is initially believed to be the main antagonist, while Courtney is revealed to have been institutionalized for her mental health issues. The story unfolds to show that Courtney assumed Alison's identity after her disappearance, leading to a series of shocking events. Ultimately, Alison returns, and the tension between the twins culminates in tragic confrontations and revelations.