My brother says I should get treated because once in a while I get drooping eyelid? Is he right?

Depends. There are many reasons for droop of the lid. First is this recent or a slow evolution of long standing? Your ophthalmologist can help discover why it droops. Treatment is only necessary if it is persistent and a health or cosmetic problem. If you are not bothered by it and have no other symptoms, then you can ignore your brother.
Yes. Variable drooping of an eyelid may be caused by myasthenia gravis. It may also be congenital and vary with time of the day, tiredness. You should have it checked.