Hi, If you haven't gotten an answer yet, I will make a stab at it. I have three half-sisters because they were born during my mothers previous marriage BUT the same thing could be true if it were my father. Half-siblings share a common parent----- either the mother or the father. Step-siblings are related by marraige only. If a woman with a child (or children) marries a man who also has a child or children, then those children are step brothers and sisters. I hope that is clear. Margaret in Nv