Cortical dysplasia is a term used to describe an area of the brain which does not form properly. It typically is only abnormal at the microscopic level. It is a common cause of seizures, and depending on how extensive the area is, can cause problems with cognition and behavior.
Without evaluating your niece, I can not tell you what to do with her behavior or treatment. What I can recommend is to have her evaluated by a pediatric epilepsy specialist. If you are in the area we have 4 excellent physicians. I would talk with specialists, like we have, about medical and surgical therapy. This is a common problem which we evaluate at the Cleveland Clinic, and have extensive experience with epilepsy surgery.
Obtaining the best seizure control possible will help with the other issues. Good luck, and sorry for the delay.