A wife's husband's "other woman" known to the wife; whether openly acknowledged or not.
The legal wife of a woman's, generally a mistress, long-term boyfriend/lover/mate.
Two or more women who share a romantic relationship with one man, but do not share a dwelling/residence.
After spending 25 years with me and simultaneously spending 18 years with her, she was more like a step-sister-wife than my husband's mistress.
Since Warren Buffett never divorced his wife Susie, she and his long-term partner, Astrid, are, for all intents and purposes, step-sister-wives.