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
Every relation had a different name, including if they are older or younger than you, and if they are male or female.
Every language that has separate terms for relatives is an impenetrable mess.