To see your sister in your dream symbolises some aspect of your relationship with her, whether it one of sibling rivalry, nurturance, protectiveness, etc. Your sister may draw attention to your family role. Or the dream may also serve to remind you that someone in your waking life has characteristics similar to your sister. Alternatively, your sister may be a metaphor for a nun. In this case, she may represent some spiritual issues.
If you do not have a sister and dream that you have one, then it signifies feminine qualities that you need to activate or acknowledge within your own self. Pay attention to the actions and behavior of your dream sister.