I wonder how close she was with her half brother and much he really knew about what was going on her. Also where was her mother in all this?
The length of time she was gone after they thought she ran away just seems odd that none of the adults decided to look for her or report her to the police as missing. Her Aunt and Uncle seemed to care enough to adopt her, yet they were dismissive of her feelings later on. Telling her to learn to love her father?
I feel like she probably felt like an outsider in her own family, an outcast. I imagine if the boy came along and gave her the attention she was seeking she could have run off with them possibly. Maybe a guy offered to drive her and took advantage of the situation.