The Cullen children heard every word, as they had been listening out for any signs of Edward's control breaking. Jasper and Emmett, who were sharing that class at that time, glanced at each other in wonder. This simple, sweet smelling, human girl had managed to get their brother to speak.
They met him in the car, half expecting him to speak. He didn't though, so neither did they. They watched and listened as he spoke to the girl, however, savouring the sound of his voice. Then he saved her life, and exposed them.
He spoke to his family then, arguing with them, defending the girl he was learning to love, and he never stopped. Edward never went silent again.
Then the girl worked out their secret, and Edward argued with Rosalie, laughing at her thoughts as she snarled at him. But inside she was smiling, revelling in the sound of her brother's laugh.
None were more relieved than Carlisle and Esme though, so when Edward brought her home for the first time they welcomed her like she was already one of their own. Bella was loved for saving their loneliest and most broken family member, and completing them all.
Bella may never know this, but when she stumbled into their loves the Cullens didn't just gain a sister, but they regained a brother too.