How is Meg's relationship with Charles Wallace important for her ability to free him?
As the novel establishes from the very beginning, and as Meg says, she's the one who is closest to Charles Wallace, and while she doesn't fully understand him, he understands her. Her father has been gone for so long that he doesn't know Charles Wallace well, and their friend Calvin has just barely gotten to know him. At the core, what saves Charles Wallace is love: Meg loves Charles Wallace very deeply, and this love is what is needed to free Charles from IT. As Meg expresses her love for him, calling him her treasure, her darling and her dear, she is able to break the grip of IT over his mind and together they can flee, landing back on earth in the Murry vegetable garden for a happy ending.