What relationship are your grandchildren to your sister?

Answer 1

Your sister-in-law is either the sister of your spouse or the wife of your brother.

The parents of the sister of your spouse are also the parents of your spouse and so are your mother-in-law and father-in-law.

The parents of the wife of your brother are not related to you.

Answer 2

For a female grandchild to your sister/brother: great niece

For a male grandchild to your sister/brother: great nephew

Answer 3

He would be your brother-in-law.