What is the correct term for a relationship between a sister and a neice?

Question:

What is the correct term for a relationship between a sister and a neice?

I have a little sister and she is six. I also have a neice ( my husbands sister's daughter) and she is 11. They get along well and love to visit each other.
I was wondering that the correct term would be for the family relationship between a maternal sister ( mine) and a materinal neice ( his). Like I know if I had a child they would be my sisters neice/nephew but what if my sister in law has a child what would she be to my sister.
She use to call her cousin, and my neice use to ask me all the time what my sister would be to her.. and I didnt know.

Any ideas?

Best Answer - Chosen by Asker: Technically, there is no relationship title between your sister and your husband's niece. There is no relationship title between your sister and your husband's niece. If you sister-in-law has a child there is not relationship title between the baby and your sister.

However, in some families that are close, they continue to call the family members the same as if they were the blood relation or the in-law. In that case, if your sister-in-law had a child then your sister could call the baby her niece.