When is a sister or brother your stepsister or stepbrother?

Answer 1

A stepsister or stepbrother is one with whom you do not have any blood ties. You are only related by marriage. If your mother marries a man who already has children of his own, they are your stepsisters or stepbrothers. That also makes the man she married your stepfather, and your mother is then stepmother to his children. If your mother and stepfather then have a child together, then that child will be your half brother or sister.