What is the relation between the two wives of a man called?
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Amongst polygamist American sects, apparently the term sister-wife is used when several women are married to the same man. These women are not actually sisters by blood, but more like sisters-in-law, and they refer to each other as sister-wives.
There’s even a “reality TV” program about one such plural family, named Sister Wives. From Wikipedia:
In the Old Testament, they are called rival wives (Leviticus 18:18, NIV). KJV uses a descriptive phrase "to vex her."
There is a description of the marriage between a man and two sisters: a sororate marriage. This word is close, but perhaps not exactly what you are looking for. If you called the relationship "sororate," you would probably be understood.