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,...
Your sister's sister-in-law is your sister-in-law, but only if your sister's sister-in-law is the wife of your brother. If your sister's sister-in-law is the sister of your sister's husband, then she...