From sister + -out-law, by analogy to sister-in-law.
sister-out-law (plural sisters-out-law)
- The sister of one's boyfriend or girlfriend.
- Girlfriend of one's brother or sister.
2008, John Crawley, Among the Aspen[1], →ISBN, TDrcNfNKVH4C, page 114:I, on the other side, was ready to strangle my almost sister-in-law. (My sister out-law.)
2013, Howard Balloch, Semi-Nomadic Anecdotes[2] (Biography & Autobiography), →ISBN, jwc6BgAAQBAJ, page 59:This included Prunella Tuff, my “sister-out-law”, a term we invented to describe the relationship between us as she was the sister of my brother in-law, Tim Tuff.
2014, Janice Ryan Hall, Deadly Ties: Sequel to Blood Ties[3], volume 2 (Fiction), →ISBN, D1dXDQAAQBAJ, page 565:“If you're openly gay, wouldn't that make you his sister-out-law?”
2015, Katherine Woodward Thomas, Conscious Uncoupling: 5 Steps to Living Happily Even After[4] (Family & Relationships), →ISBN, 2pv0BQAAQBAJ, page 59:I, on the other side, was ready to strangle my almost sister-in-law. (My sister out-law.)
- The sister of one's ex-spouse.