A more thorough explanation:

Propior sobrino

Propior sobrino is a legal term that refers to the son of a great-uncle or great-aunt, whether on the father's or mother's side of the family.

For example, if your father's uncle has a son, that son is your propior sobrino. Similarly, if your mother's aunt has a son, he is also your propior sobrino.

These examples illustrate how propior sobrino is used to describe a specific familial relationship. It is a term used in civil law to identify a relative who is not a direct ancestor or descendant, but rather a cousin once removed.