Meg broke IT's control with what?

In Madeline L'Engle's novel A Wrinkle in Time, the protagonist, Meg Murry, ends up battling the evil powers of IT, a monster that largely resembles an oversized brain. This antagonist seeks to control and manipulate the fictional land of Camazotz. Assisting her in this battle against this force of evil are her brother (Charles) and her school friend (Calvin O'Keefe). 

Charles Wallace ends up possessed by IT's power, entranced into believing that IT is good.  He tries to convince the others that IT might be good. Charles says,

And that's why everybody's so happy and efficient . . ..  nobody suffers here . . . nobody is unhappy.

Ultimately the children are unconvinced: they continue to realize IT's evil powers, despite Charles' arguments.  Calvin declares,

. . . But you're right, Meg, and Mrs. Which is right. This is Evil.

Soon after Calvin asserts that IT is wicked, the children discover where Meg's father is being held captive:

He moved rapidly down the corridor and again held up his hand to make the wall transparent. They looked into another room or cell In the center of it was a large, round, transparent column, and inside this column was a man. "Father!" Meg screamed.

Calvin and Meg work together not only to regain Charles from his mental possession, convincing him rightly of IT's evil, but also to save the children's father. 

For several chapters, IT has captured Charles. In Chapter 12, Meg shouts, 

But it's Charles Wallace! IT has him, Mrs. Whatsit! Save him, please save him!

Mrs. Whatsit responds that she can do nothing to help in Camazotz. The situation looks bleak and hopeless. The children have to find the answer to defeat IT on their own. 

At this point, Meg ponders the power of love and hate. She realizes that IT abounds in hate. Charles Wallace, still possessed by evil, tells Meg that Mrs. Whatsit hates her. She knows that this is not true; she's experienced Mrs. Whatsit's love. The idea of this great love helps her to realize that love is far more powerful than hate. She continues to ponder the most powerful relationships she'd ever experienced:

Love. That was what she had that IT did not have. She had Mrs. Whatsit's love, and her father's, and her mother's, and the real Charles Wallace's love, and the twins', and Aunt Beast's.

Once she determines the power of love, she realizes that all she has to do is love Charles Wallace, despite his possession. She determines that she will love him, no matter how unworthy of love he seems in this moment:

Charles. Charles, I love you. My baby brother who always takes care of me. Come back to me. . .

She repeats, again and again, how much she loves him. Her emotions begin to overwhelm her, and she ends up crying as she pleads for his right state of mind to return. 

Finally Charles Wallace returns to his normal self:

Charles Wallace was crying out, "Meg, Oh, Meg!"

At that point, they embrace each other, a physical demonstration of the great, powerful, and uniting power of love. Their tears change into laughter; victory is won. The power of love ends up being a major theme (and a moral lesson) in the story. The idea of love versus hate, or good versus evil, is a central conflict. Meg wrestles with this conflict internally, as she seeks to love her brother when he is possessed. Overall, the conflict of good versus evil is an external conflict, as IT attempts to oppress all good in his world. The conflict is resolved when Charles returns to his right way of thinking due to the power of love.