co-aunt

plural co-aunts

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

   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

   2008 August 4, “Camp Jean”, in The Favorite Aunt: Not that it’s a competition, archived from the original on 24 Mar 2009

   Synonyms: co-sister-in-law