What does it mean when your sister dream that you going to have a boy?

Dreams are about the dreamer's own emotions, thoughts, urges, etc. So in her dream, "you" might represent your sister, and her own feeling about having a child. Or, her dream might be a metaphor showing how she feels the need for someone younger to love and care for, but tempered by her feeling unready for children of her own. There are endless possibilities, but you can be certain that your sister's dream does not mean you are pregnant. Or, if you are pregnant now, her dream does not determine the sex of your child. An Ultrasound can settle that matter.