What Relation Is My Sister's Mother-in-law To Me?

In Chinese, "in law" is also a relative, but differentiated by adding qualifying words before the title. So, a man's sister will call the man's mother in law "yan1" aunt (Cantonese dialect) or "yin" aunt (Mandarin).
Chinese family terminology is very complicated -your father's elder brother, his younger brother, and your mother's brother all have different titles, unlike in English where they are all uncles.

She simply is just that. "your sister's mother-in-law" still deserving of some type of respect though. By all means. Hope this helps.(smile)

There is no formal relation ship between you.
Though sometimes, in order to pay respect and to show that there some emotional, family related ties, in Suriname we use to say "aunt" in situations like this.