SIL
Also called sil, SILs, sister-in-law, sisters-in-law
SIL stands for sister-in-law, and parents use it on either side of the family: a partner's sister, a brother's wife or the wife of a partner's brother. It belongs to the same shorthand family as MIL, FIL and BIL.
How it's said
My DP messaged his mum and asked who it was and she said it was his sister (SIL).
Thankfully me and my SIL get along extremely well, we're very similar in mindset and I liked her from the minute BIL introduced us.
...I would take DS for a drive (sometimes with the windows down!) or have dinner with my SIL, or just slowly walk around the complex.
Who says it
Where it came from
Sister-in-law comes from Middle English suster-in-lawe. The -in-law ending marks a relative by marriage, which the law calls affinity, rather than by blood. Cambridge Dictionary counts a brother's or sister's wife, a husband's or wife's sister and the wife of a spouse's sibling as sisters-in-law.
Guides to parenting forum shorthand list SIL beside MIL, FIL and BIL. Wiktionary also records it for sibling-in-law.