The best way to check if your sister is really who she claims to be is through a DNA test. However, realizing the expense of such a test, you can fall back to birth certificates, public records, photo IDs, or asking your mom or relative.
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 sisters sister-in-law is: the sister of her husband, and not related to you at all, or the wife of her brother (i.e. of you if you are male, or of your brother) and is then also your...