Wiktionary
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.