I got into an argument with some coworkers for calling my biological brother's (then) ex-wife's brother my brother-in-law.

So I agree with you, but I guess other people are a bit bigger on semantics than we are.

But what about father-outlaw and mother-outlaw?

I'm not married and I never was. And on paper I still have in-laws.

I met this girl around 2008 and we became friends with benefits. Suddenly 17 years later we have mortgage, kids, pets, and everything else together. Some might go as far as saying that it's starting to get serious between us.

Anyway, due to having had shared address and offspring we're legally considered married for all practical purposes.

Although I've heard that in French, stepmother and mother-in-law are the same word. Which I can see the logic behind. One is your spouse's parent and the other is your parent's spouse

Thats hella deep

isnt my brother's wife's mother my mother-in-law?

One of you or your siblings has to be married.

It otherwise doesn't work: brother/sister in law wouldn't exist. Unless siblings marry siblings, there's no other way to create that link if you all have to be married.