If your sister-in-law is your wife's sister, her sister's daughter could be your daughter, whom you may not marry. She may also be the daughter of another sister-in-law. Then you could marry her, but...
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...
A sister in law, by definition, is the sister of your wife. As long as you aren't still married to your wife, it would be legal. A sister-in-law can also be your brother's wife. You can marry her...