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 phrase is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited.[in-law etymology, in-law origin, 英语词源]