Sisters are someone you can never have sex with. So a sister (for men especially ) can symbolise a relationship where sex is out of the question. It maybe that.
Do you have a good relationship with your sister? Do you relate to her? Do you feel comfortable around her? Do you find it easy to talk to her? Do you respect.
In the past month, I've had three sex dreams involving my best friend's sister. We haven't interacted that much, but I do enjoy talking to her. I will admit to finding.