She was 15 years old, not 7, a very large amount of girls loses their virginity between the ages of 14 and 16. And yes, I believe what she said first time. Maybe, her brother's reaction was the reason that she changed her story? She thought that he will accept her with her past?
And furthermore, the story takes place in US, where young and beautiful female teachers are jailed for having sex with male students. She could tell about everything, I doubt that she would've been disgraced for it in Western society.