Technically she is no relation to you and your family. You should just make up a term. (I call my SIL's best friend my "best friend-in-law.")
I would like to lodge a complaint about the English language, though: My brother's wife (if he had one) would be my sister-in-law. If my husband had a sister, she would be my sister-in-law. My husband's brother's wife is my what? My sister-in-law.
How can three different relationships be called the same thing? It's LAME. How would anyone know which one I'm talking about?