It's probably because it's often used to categorize any relative romance either by marriage or blood, in Japanese there's a term "Inseki" that means "relative by marriage" which if I remember correctly is often used to categorize step sibling romance (I might be wrong though.) And while English has "step-sibling" I personally wouldn't exactly call it a "term" since it isn't a new wordQuestion: if they're just step siblings....then why tf does this have an incest tag? That's not how that works
not related by blood