Call her by her name.
you call her your daughter's husband's sister
Your brother would call your husbands sister by her first name. Or, "my (sibling's) sister in law.
brother in law
Mom
Sister-in-Law
I think that you call them your cousin in law because its pretty much the same concept as a sister in law... its your husbands cousin so it would be a cousin in law
sister in law
Your Sister-in-law
No. An Uncle is the brother of either your Mother or Father. You're kids may call him 'uncle'
Your husband's sister's kids are referred to as your nieces and nephews by marriage. More specifically, you can call them your "nieces" if they are girls and "nephews" if they are boys.
You might want to call her sister-in-law, even though to the best of my knowledge there is no officially designated relationship for this situation. But I am certain that she will be very happy to be accepted with the above designation.
I would say aunt in law. If I'm reading this right, you were introduced to your mother.