Definition from Wiktionary, the free dictionary
co-aunt (plural co-aunts)
- 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.
- a woman's sister, as aunt in common to the children of a third sibling
2008 August 4, “Camp Jean”, in The Favorite Aunt: Not that it’s a competition:
- A few years ago, my sister and co-aunt Judy started flying [our niece and nephew] Violet and Dash down to California to spend a week with her
- (sibling's spouse's sister): co-sister-in-law (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)