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I am sorry to hear about the problems you are experiencing with your brother. It can be very difficult to see such symptoms in someone you care about. Your brother is lucky to have such a concerned brother in his life, even if you are not living in the home. While I certainly cannot diagnose your brother without seeing him, I might suggest he be evaluated for a condition known as Aspergers. Some of the symptoms that you describe are suggestive of this disorder, while others are suggestive of possible Attention Deficit Disorder, Learning Disorder or anxiety.
The following link to the Autism Society of America may be helpful to you in locating resources: http://www.autism-society.org/site/PageServer?pagename=life_aspergers
Aspergers is a pervasive developmental disorder that is in the Autism spectrum. Symptoms include problems with social skills, repetitive behaviors, unusual preoccupations or rituals, difficulty with communication, limited interests, and coordination problems. Despite these problems many patients with Aspergers possess particular skills or talents.
Given what you have described, I would encourage you to speak to your mother regarding scheduling a meeting with a psychologist who specializes in Aspergers to either rule out or obtain an accurate diagnosis of your brother's condition. Whatever the issue is, I would doubt that it is simply due to you and his sister leaving home.
I hope this answer is helpful. Please let me know if I can clarify further.