I understand your concern for your sister. There seems to be something wrong with her. The episode is called Syncope
The causes can be related to heart, it can be neurological cause or a cause of decreased blood supply to the brain.
The possibilities are carotid artery
insufficiency, coarctation of aorta, severe aortic regurgitation
, neurological causes may include a space occupying lesion, epilepsy
or seizures. Other common causes include anemia, nutritional deficiencies.
Please speak about these with her treating doctor.
Hope this helps.