My little sister (11 years) sees total blackness for 10 seconds while sitting or walking doesn't matter maybe twice a day, why is that ??

She needs to see a doctor: Blacking out for short periods of time is not common, or normal. She needs to see a pediatrician or Dr. who can evaluate her further, to make sure that she's not having blackouts from seizures, or from poor circulation to her eyes, or brain. S/he can do a full Eye and physical evaluation and let your sister and you know what to do next. Also, it may be related to her diet, or not eating enough food. Don't delay!