Your brother-in-law's brother. Though many will refer to them as a brother-in-law, they technically are not related to you.
If the brother-in-law's daughter is also your sister's daughter, she would be a niece. Otherwise, there is no relationship.
It's brothers-in-law.
After your brother dies, his wife is still your sister-in-law. If she remarries, you or she may find it convenient to think of her as your former sister-in-law.
Your grandson is your brother-in-law's great nephew, and your brother-in-law is your grandson's great uncle.