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sister-wife

n. A woman who is simultaneously sister and wife to her husband:

Usage examples of "sister-wife".

If Dorindha will accept me as her sister-wife, I will make my bridal wreath to lay at Bael’.

My sister-wife Ishtar is director of the rejuvenation clinic and chairman of the board of the medical school, and still finds time to teach.

But any of my sister-wives would supply womb room if she didn't happen to be pregnant.