To think it's not weird to call someone 'The In laws'

[SoleTraderLady]

In this situation?

My sister and her partner of 3 years. They do not live together and are not married but very happy together. I referred to them as the inlaws and was told that isn't the right term for them.

@Gullible2021 there’s a difference between a committed relationship and a casual one, that in the past was broadly delineated by marriage but isn’t necessarily anymore. I think it would be useful to have more language to indicate this.
In law expresses a degree of entanglement and obligation. It’s a type of family. But “girlfriend’s family” doesn’t express whether that level of interaction has been reached.

I also think it would be useful to have words to describe eg the relationship between myself and my brother’s wife’s brother.

[quote thelegohooverer]@Gullible2021 there’s a difference between a committed relationship and a casual one, that in the past was broadly delineated by marriage but isn’t necessarily anymore. I think it would be useful to have more language to indicate this.
In law expresses a degree of entanglement and obligation. It’s a type of family. But “girlfriend’s family” doesn’t express whether that level of interaction has been reached.

I also think it would be useful to have words to describe eg the relationship between myself and my brother’s wife’s brother.[/quote]
I’d describe “Girlfriend” as casual. Don’t live together, or share bills or responsibilities.

“Partner” as serious, long term living together, sharing financial outgoings, next of kin etc.

I agree with the last sentence though. Never sure how to describe my Sister-in-Law’s sisters in relation to my own relationship with them.

Are you just looking for a nice shorthand term to describe them in an affectionate manner? Sister and partner: “The Sappies?”, Brother and partner: “The Bappies?” - just ask them what collective noun they want to be called and take it from there!

Maybe because they don't live together and aren't married your dsis doesn't see them as inlaws yet.
My dh parents were just that until we got married, then they became pil. Or better still, the outlaws.

Your sister isn’t and won’t ever be your sister in law, that would mean she was your husband’s (or partner‘s sister.

Ignore me I didn’t read the OP’s update properly.