Where does SIL come from?
According to Green's Dictionary of Slang, the slang term sil dates back to the 1920s, referring to something or someone who is "silly" or "foolish." In the 1930s, it was used in gay slang for an infatuated gay person. Both senses had some currency up until the 1970s.
Sil, along with the perhaps more common sill, re-emerged online by at least 2005. Like in the 1920s, it became a cutesy way to shorten the word silly when describing someone who is acting in a goofy or fatuous way. The internet shortening may be part of a larger trend of shortening words (e.g., totes (totally) or jelly (jealous)).
By the launch of Twitter in 2006, SIL was more commonly being used as a convenient acronym for sister-in-law (a wife or husband's sister) and son-in-law (a daughter's husband). This SIL joins the family or MIL, FIL, and BIL: all stand-ins for mother-in-law, father-in-law, and brother-in-law, respectively. Urban Dictionary included an entry for sil for sister-in-law in 2008, noting the acronym as part of SILF, vulgar slang for sister I'd like to fuck and modeled on MILF.
Who uses SIL?
As a shortened form of silly, sil is more commonly seen as sill on social media, where it often has an affectionate or lighthearted connotation.
SIL is still used in social media and popular online for sister-in-law and son-in-law for its convenience, often conveying a sense of intimacy and familiarity. While diverse men and women have used sil on Twitter this way, women have seemed to favor it for sister-in-law. However, it seems women tend to use it more frequently.
Since Donald Trump's presidential campaign and election in 2016, SIL has become increasingly used to refer to his controversial son-in-law and adviser, Jared Kushner.
Why would Sam put her sunglasses on to drive home?! It's nighttime cmon Sam don't be so sil
this weekend I told my SIL I have a wireLESS dachshund instead of a wirehaired dachshund. my dog can go everywhere!
You may think that not having your SIL present will decrease your stress and anxiety, but the truth is you won’t really be able to relax knowing that your MIL will be wondering why her daughter wasn’t invited to the party.
HuffPost, December, 2014