Looking on the internet deeply has found these results:

sister-in-laws is the most popular phrase on the web.

co-sisters 132,800 results on the web	sister-in-laws 1665,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
I figured a co worker or a sister, not wife.	Maria, is it true that in your sister in He's your husband's adoptive father and your sister laws and hanging out with Jews together. in laws husband. Besides, we're both dating Puckermans, and someday we'll be sister in

co-sisters

132,800 results on the web

sister-in-laws

1665,000 results on the web

More popular!