To see your sister in your dream, symbolizes some aspect of your relationship with her.
If you two have different relationships with your parents, your dream may be calling attention to your feelings about that.
Or, someone you just met could have qualities similar to your sister.
If you do not have a sister and dream that you have one, then it signifies some qualities that you need to find or develop within your own self.
Pay attention to the actions and behavior of your dream sister, for they are your biggest clue about what those qualities might be.