Co-mother

It is considered incestuous for a co-mother and co-father to have sex relations

Source: wiktionary

the two of them went to their mother's forest hut and tried to find a way to avenge their mother's death and kill their co-mother.

Source: wiktionary

Notwithstanding the voluminous “co-wife” literature that Western anthropologists have used to define African marriage, “co-mother” is the preferred idiom in many African cultures for expressing the relationship amongst women married into the same family.

Source: wiktionary

The dilemmas engendered by the absence of a biological tie between a child and co-mother illuminate the centrality of familial rights and obligations in American kinship.

Source: wiktionary