in law

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 phrase is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited.