Each religion has a set of rules that govern how the followers of that religion should behave.
These rules, put together, are called religious laws, e.g. Islamic Sharia law, Jewish Halakha, etc. and they include ethical & moral codes relating to different aspects of life.
In Christianity, there is Canon law that outlines rules to organize and direct activities in the Catholic, Anglican, and Orthodox churches.
A set of these rules in Canon law governs relationships.
These rules recognize that there are relationships, which are natural (e.g. father, mother, sister) and there are relationships that are not natural but are still relationships in the eye of law.
And these ‘not natural’ relationships were described in the Canon law as relatives-in-law.
The earliest mention of in-law in English was in the 14th century andtill the early 1800s, the term was used to describe all ‘not-natural’, ‘non-blood’ relatives that the Church prohibited you from marrying in case your spouse died.
So, your spouse’s brother, sister, parents, children were in-laws and so were your stepfather*The term father-in-law was used both for spouse’s father and stepfather., stepmother, stepbrother, and stepsister.
And since in-laws were off-limits as far as marrying was concerned, a man could not marry his deceased wife’s sister or a woman her dead husband’s brother.
The position changed in the 1800s for unknown reasons, with the in-law shifting to relationships created by marriage and step- starting to be used to describe old relationships that existed between one’s partner and another person.
While most religious laws today are distinct from civil laws and focus on internal workings of religious bodies, they had the force of civil laws in the past.
So, it was well within the power of Church leadership (under Canon law) to decide the legality or illegality of a marriage.
The law in the in-law then is the Canon law and not civil law.
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