Your sister would be her great aunt and she would be your sisters great niece.
second cousin
Your great grandmother's sister is your great great aunt. Her great grandchild is your third cousin.
Your great grandfather's sister is your great great aunt. Her grandchild is your parent's second cousin and your second cousin, once removed.
Your daughter and your sister's child are first cousins. Your daughter and your sister's grandchild are first cousins, once removed.
Second cousins.
The grandchild of your grandmother's first cousin is your third cousin. The grandchild of your grandmother's second cousin is your fourth cousin. The grandchild of your grandmother's third cousin is your fifth cousin.
Your great uncle's grandchild is your second cousin.
Your sister's great grandchild is your great great niece or great great nephew.
The grandchild of your great grandmother's sister is your second cousin, once removed.
auntie .
The common ancestor is your parents. Your children and your sister's children are first cousins. Your grandchildren and your sister's grandchildren are second cousins. Your great-grandchildren and your sister's great-grandchildren are third cousins.
Your are a great uncle. I mean, I'm sure you're a wonderful uncle! But great uncle is your relationship. The child is your wife's sister's great grandchild, right? So you are great uncle.