Wiktionary
co-mother
n. (context obsolete English) the relationship of a godmother to the other god-parents, and the legal parents, of a child.
Usage examples of "co-mother".
Their birthmother was dead, murdered by Falconers or something like, it seemed, and Huana their co-mother wanted it made clear that her sister-friend was to be remembered.
She was one of my co-mothers, a medical technician who specialized in psychology.