Your stepsister is a girl or woman who became your sister because a parent of yours married a parent of hers. You two share no blood at all — her mother married your father, or her father married your mother, and now you live in one family. This is the key thing: a stepsister and you have no parent in common. That is what separates her from a half-sister, who shares one real parent with you (the same mother, or the same father). English keeps those two words apart carefully, and it matters to people who have them. Most English speakers know the word first from Cinderella, where the stepsisters are cruel — so the word carries a faint bad smell in stories that it does not deserve in real life. Real stepsisters are often close, and many people simply say 'my sister' and skip the step- entirely once the family has settled. You would use the full word 'stepsister' mainly when you need to explain the family clearly to someone outside it. It is written as one word nowadays; 'step-sister' with a hyphen turns up in older books and is not wrong, just old-fashioned.