The sister in dreams usually represents the feeling side of ourselves. We have the ability to make links with that side of ourselves through being able to understand our sister’s personality.
In a man’s dream if she is older, the sister can represent the potential for persecution, but also of caring.
If she is younger then she can epitomize the more vulnerable side of him.
In a woman’s dream if the sister is younger, she can represent rivalry.
If older she stands for capability.