[in-law] 
| a relative by marriage. |
"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.
