Eyelid dropping, or as its medically known: ptosis, is a common condition. It can represent either a normal part of aging, however can also represent a serious situation. I would strongly suggest that you have your mother see her doctor
in order to evaluate the situation.
Ptosis occurs when the muscles in the upper eyelid are not strong enough to hold open the eyelid. There are many causes of nerve damage to the nerve that innervate those muscles. Nerve damage from diabetes, trauma are common. There are also concerns of a tumor pressing the nerves, so this can rarely represent a brain tumor.
Another condition that can causes ptosis on both sides in myasthenia gravis
. This is a disease characterized by general weakness and fatigue.
Another classic situation where ptosis is present is part of Horner's syndrome. This is where ptosis occurs along with both a small pupil in that eye and a decrease of sweating on that side of the face. This is often because the specific nerve is cut off -- a nerve that starts in the chest so can be caused by lung cancer. If this is the case, please tell your doctor.
Talk to your doctor. This can be just weakening of the muscle with age, but could be a serious problem.