Is "in-law" (meaning related by marriage) a modern usage?

A language usage nitpick that's been bothering me. In 16th/17th century documents, I've seen the word "sister" refer to a spouse's sister or a brother's wife, what we would now call a "sister-in-law". I've also seen "son-in-law" used to mean a stepson. Does anyone know when the modern meaning of "in-law" became standardized? Would it seem anachronistic to use "sister-in-law" in the 17th century? Should I just say "sister" or "husband's sister"? Am I overthinking this?

Thanks. Looks like "sister in law" is fine for my time period, but "in-laws" isn't. My two SILs are also good friends, so they might refer to each other affectionately as "sisters" once their actual relationship has been established to the reader.

I also found "good sister" as a Scots term (https://en.oxforddictionaries.com/definition/good-sister). I can't use it because my novel is set in Ireland/England, but someone else might find that useful.

It also looks like "X in law" could have meant any relationship by marriage, including what we would call step-X. Although "stepbrother" and "stepsister" existed in the 15th century and "stepmother" goes back to at least the 1300s. (https://www.etymonline.com/word/step-) Possibly the two formations lived alongside each other until the modern distinctions emerged. I'll stick with the modern usage so as not to confuse readers by referring to a young child as a "son in law"!

I have bookmarked the etymology.com link for future reference. Thanks amergina!