Definition from Wiktionary, the free dictionary
co-sister (plural co-sisters)
- colleague ("sister" in the same craft, occupation, or society)
1900, Arthur Francis Leach, Report on the manuscripts of the corporation of Beverley, page 91:
- At the death of the co-sisters of the Guild, every co-brother of the Guild to be present
- (India) One's husband's brother's wife.
1989, Abusaleh Shariff, Fertility transition in rural South India, page 263:
- During this time I came to know about tubectomy from my co-sister and also from the female health worker. This time both my husband and father-in-law refused [me the surgery] on the ground that I was the eldest daughter-in-law and a responsible person for the whole family.
- co-sister-in-law (in the more common sense only)