Hi my 5� year old sister rolls her eyes back into the head since she was like 3.please tell me what is it?

Hi my name is Gabriel and my 5� year old sister Lorena has been rolling her eyes back into the head since she was like 3.I am sure she does not do this on purpose.When she does this it lasts like 4-6 seconds and like 12-20 times a day,she does this at anytime a day no matter what she is doing and when she does it she lightly moves or shakes her arms.Me and my family did not worried about because we read that it was something that she would stop doing it when she grow to be 5 or so.Can you tell me whats the best thing to do,I ill be very thankful if you do.Thank you a lot.