co-aunt

Noun

 * 1) of a woman, an aunt of one's niece or nephew who is not one's sister; a co-sister-in-law as common aunt to the children of two women's married siblings.
 * 2) a woman's sister, as aunt in common to the children of a third sibling

Synonyms

 * (relative to the married siblings rather than to the children; however, this term generally refers to a different relationship, that of two women who marry brothers)