Your sister's father-in-law is not related to you and neither is his sister.
Sister in law
Your sister's son is your nephew.
Your sister's daughter would be considered your niece.
If he is your brother in law then his sister is your sister in law. So your sister in law's daughter would be your niece.
There is no relationship. You do not share a common ancestor. The connection is simply through marriage.
In the genealogical sense, the English language has no relationship called "great sister."
Your granddaughter and your sister's grandson are second cousins.
Your aunt is the sister of either your mother or your father. Your aunt's sister is either:your mother, and her daughter is you (if you are a girl) or one of your sisters, oranother one of your mother's sisters, and her daughter is your first cousin, or,one of your father's sisters and her daughter is your first cousin.
"A sister is both your mirror β and your opposite." β Elizabeth Fishel "Sisters are different flowers from the same garden." β Unknown "Having a sister is like having a best friend you canβt get rid of. You know whatever you do, theyβll still be there." β Amy Li
Your sister's child is your nephew if it is a boy, or your niece if it is a girl.
If this is a question then no, that is not always the case
Your sister's husband is your children's uncle. They are his nieces and nephews.