Although the stand alone phrase 'in-law' does indeed date from the mid 19th c. it's an abstraction from much older specific phrases such as 'father-in-law' or 'brother-in-law'. The earliest recorded such usage is 'brother-in-law' from 13th c. English Canon law where it's used to outline who may marry whom (To give a very famous illustration from a few centuries afterwards; think of Henry VIII arguing for his marriage to Catherine of Aragon to be annulled on the basis that she was his sister-in-law whilst his brother Arthur was alive, although he did not use term itself but instead quoted some relevant bits of Leviticus)

The form begins referring to sibling in-law relationships but is soon extended, for example by the 15th c. we have 'mother-in-law' Modyr in Lawe in this example being used.

In-law has had different meanings too. In pages of the 19th century censuses of England I've seen what we would call stepchildren called sons- and daughters-in-law (I have also seen them called stepchildren)