>>719554
What you're describing is half-sister, not step-sister. A step-sister is when your father or mother marries someone who already has kids of their own. If your father marries a woman who has a daughter from a previous marriage, then that is your step-mother and step-sister. So, she is not your mother's daughter.
I don't think the Bible says anything about step-siblings, but admittedly it's not something I've ever really bothered to look for.