well she could have ADD, but with what you mention about the stealing i wonder if it is more of an OCD.... OCD can take on many forms, including kleptomania, an obsession with stealing things...
people with OCD also tend to have obsessive thoughts about things which could account for her school problems.... ADD and OCD commonly occur together also... i'd do some research, maybe even look for the DSM IV checklists for ADD and OCD and see if it suits your sister and show it to your mother...
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