Hi, While the conditions may be associated with each other it s not likely to be mistaken for each other. Borderline personality disorder is manifested by emotional instability and impulsive behavior, altered patterns of thinking, difficult relationship with others etc., but it doesn t refer to feeling uncertain about ones identity or showing different identities like in multiple personality disorder. If you feel that is the case you should express your concern to the psychiatrist. Perhaps your brother has never manifested that side during the psychiatric interview, interviewing family members is often an important part of making a psychiatric diagnosis. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Olsi Taka, Neurologist