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 daughter of one’s cousin or cousin-in-law
A daughter of one’s sibling, brother-in-law, or sister-in-law; either the daughter of one's brother ("fraternal niece"), or of one's sister ("sororal niece").