Do the signs point towards aspergers syndrome?
I work with autistic children (7-11 years) and from your description your step-brother may have Asperger’s Syndrome. Autism and Asperger’s both require medical diagnosis, so if anyone other than a Doctor or Psychologist tells you otherwise, they are not qualified to make that diagnosis. Maybe suggesting to Mum that additional help might be available would an opening. Often parents of children with special needs find it difficult to accept their child is special.