Oo, that's such a good question.
I decided to look it up to see what people are using. One forum group listed acronyms and whilst keeping Sister-in-law as SIL, then assigned SNIL to Son-in-law.
However, the
Ancestor Search by US Census abbreviations has really covered all the bases for relative acronyms. For example:
Son-in-law as Sl
Sister-in-law as Sil
Step Son-in-law as Ssl
Step Sister-in-law as Ssil