Entries linking to sister-in-law
These are from PIE *swesor, one of the most persistent and unchanging PIE root words, recognizable in almost every modern Indo-European language (Sanskrit svasar-, Avestan shanhar-, Latin soror, Old Church Slavonic, Russian sestra, Lithuanian sesuo, Old Irish siur, Welsh chwaer, Greek eor). French soeur "a sister" (11c., instead of *sereur) is directly from Latin soror, a rare case of a borrowing from the nominative case.
According to Klein's sources, probably from PIE roots *swe- "one's own" + *ser- "woman." For vowel evolution, see bury. Used of nuns in Old English; of a woman in general from 1906; of a black woman from 1926; and in the sense of "fellow feminist" from 1912. Meaning "female fellow-Christian" is from mid-15c. Sister act "variety act by two or more sisters" is from vaudeville (1908).
1894, "anyone of a relationship not natural," abstracted from father-in-law, etc.
The position of the 'in-laws' (a happy phrase which is attributed ... to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness. [Blackwood's Magazine, 1894]
The earliest recorded use of the formation is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited. Thus the word originally had a more narrow application; its general extension to more distant relatives of one's spouse is, according to OED "recent colloquial or journalistic phraseology." Middle English inlaue (13c.) meant "one within or restored to the protection and benefit of the law" (opposite of an outlaw), from a verb inlauen, from Old English inlagian "reverse sentence of outlawry."