Hi, it sounds like your sister may be experiencing a migraine/ seizure link. There are doctors who don't know about it (especailly GPs) or discount it's existance. But it can and does happen. I have it. If I get a migraine that is very bad and I can't get my migraine med. quick enough to get fast relief I will often times have a seizure.
Are her headaches on one side, does she have nausea/vomiting, an overall "vague feeling" or tingling of any limbs. Sensitivity to light and sound are also very common symptoms.
If so they may be migraines. They are also associated with auras before a seizure.
She should keep track of her headaches and write down all symptoms associated with them. Until she can get back to her doctor there are things she can do to help with the headaches. Taking an OTC pain killer, laying down in a dark, quiet room and after the headaches eases some drink a cup of strong coffee or tea. Caffiene helps to open the blood vessels and help ease the pain.
Hope this has been some help. Feel free to ask if you have any further questons.