Blood relation

Is it a legal issue or moral issue when 2 person of no blood relation but bond by the law fall in love with each other. A man and a woman get married, and their children, each having one from a previous relationship fall for each other. So in this case, how should it be handle. Technically, I don't think the law actually such relationship but it is just very weird for all the 4 parties. The step-father and the father-in-law is the same person. The step-mother and the mother-in-law is the same person. If the parents have a child, should the new child calls the children brother and sister or brother-in-law and sister-in-law?