[troupo]

It is an objective fact. As a speaker of Russian (native), Romanian and Turkish.

E.g., in English a spouse's parents are just that, parents-in-law. And the spouse's brothers and sisters are brothers- and sisters-in-law. That is it.

Let's take Russian. Each of the following is a separate term:

- husband's mother

- husband's father

- wife's mother

- wife's father

- husband's brother

- husband's sister

- wife's brother

- wife's sister

The inverse is also true. Your daughter's husband's siblings and parents have a different term from your son's wife's.

Very, very simple.

These are variations on the Sudanese Kinship category, and they are both arbitrary and complex: https://en.wikipedia.org/wiki/Sudanese_kinship?wprov=sfti1