Is a *by-law* the same thing as an *in-law*?

An aunt-by-law would be the wife of one of one’s parents’ siblings. You share no blood with that woman, but she has married your uncle.

I suppose you could call her an aunt-in-law, but that sounds rather odd. Normally one’s in-laws are those new family members created by one’s own marriage, not someone else’s.

We never made a distinction between by-blood and by-law uncles and aunts growing up, although of course you were always aware of it.

Then there’s the old phenomenon of an uncle-by-courtesy, which is something else again. Now it’s simply an honorific for someone older than you whom you would not address by their first name, but who is a family friend, likely of your parents’.