The woman who kidnapped Ben was a class-mate of the mother–she had gone to Chicago for her High School reunion. They even saw the lady for a bit in the hotel, just before the reunion, and the police did consider her a suspect, but for some reason never followed up on it. Ben still felt more connected to his kidnapper and her husband than he did to his real/blood family; he had very few memories of them since he was 2-3 when kidnapped.
The performance of Jonathan Jackson in this movie (he was the older brother, Vincent) was excellent, IMPO. 
tarragon