My brother is having a epilepsy since 15 years ago with mental incapabilities affection as a complication, does it irreversible?

Hard to say: Seizures can be for many reasons and of different types. Some can be due to trauma or serious medical problems. If so, they may not be reversible. Other causes can get significantly better with the right kind of medication. Sometimes, mental incapacities can be from low grade constant seizures, so treatment might help. 15 yrs is a long time though, so it may not improve much. Get a clear diagnosis.