sister-wife

A woman who is simultaneously sister and wife to her spouse.

My Grecians shall victorious prove, By me led on to War, the Sister-Wife of Jove.

Ptolemy VII […] married Cleopatra, the sister-wife of his predecessor.

In a polygamous marriage, a woman who is simultaneously the sister and co-wife of another.

Enmities between sister-wives will [be,] from their having known each other too intimately all their lives, more unmannerly than where they are strangers to each other

A co-wife in general.

Yüeh-niang and her sister-wives, all dressed in formal gowns, came out from inside to greet them and ushered them into the rear reception hall

The concept of 'sisterhood' was popularly applied to the official organizations for Mormon women, and Mormon co-wives were and are still known as 'sister-wives'