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.
A sister-in-law of one's mother; the wife or ex-wife of a maternal uncle.
A sister of one's mother.
A similar great aunt, etc.