co-mother

The relationship of a godmother to the other god-parents, and the legal parents, of a child.

In polygamy, a wife of one's father who is not one's mother; that is, co-wife of one's mother.

In polygamy, the relationship between one co-wife and another, in respect to their children.

In a lesbian couple, the nonbirth mother (partner of the birth mother) of a child, especially one who takes an equal role in mothering the child.