Answer:
It depends. You may be stepsisters- your parents split up and your Dad gets a new wife and has a daughter, your sister. What if they brake up and your sister's step mother has another child? Then no, because not either of your parents are related to this baby, though your half sister from your father's previous marraige would be realted to it. Or it could be your mother. Or you could be the last sister. The second would be realted to them both and the other two only related to the second.