If you are dreaming of your own sister, she may not be playing herself in the dream, unless there is a current issue with her that is on your mind daily. Otherwise, your sister is standing in for a part of yourself that is like her. What do you and she have in common? Is that quality – or fault – being helpful to you in life right now? The way your sister is behaving in the dream is a good indication of how this particular quality or fault is helping or hurting you right now.
If you do not have a sister but dream that you have one, and you are female, then she represents your sisterhood in the female community. She represents the fact that you are not alone in your current situation in life.
If you do not have a sister but dream that you have one and you are male, then your sister is standing in for your female side, that part of you that can be sensitive, nurturing, a good listener and creative.