The sister in your dream could mean that you have many sisters to put up with. She could represent future female descendants, or she might represent things like aunts or older female children. You might be worried about not seeing her much even if she’s hundreds of miles away and enjoying a peaceful retirement on the beach.
People dream about their sisters frequently, be it about their happiness, sadness or discovery of their sibling’s secret new life. Perhaps it is a sign you need to spend more time with your sister before you both grow up too fast and start drifting apart.