To dream of your sister represents the bond that you both share. Either you are very close or you are often in conflict with each other. Perhaps your sister reflects the position you hold within your family and your feelings regarding acceptance. It may also imply that your sister and some other person you associate with share the same attributes. This dream may also be symbolizing a nun and therefore imply that you may be having emotional conflicts.
If you do not have a sister and dream that you have one, then it implies that you possess certain qualities that you have not discovered as of yet. Consider what the sister is saying and doing.