Game concept
Blood Brother
The game also links it as Blood Brothers, Blood Brotherhood, Best Friend's, Best Friends'.
From the game's encyclopedia
A Blood Brother is a special form of Friend and Ally. Characters may only have one Blood Brother: this Blood Brother becomes a symbolic member of their family (and, thus, must not already be Family). If a Blood Brother's call to War is refused, the Special Relation will break and the two will become Rivals.
Blood Brothers, their Children, and their Primary Spouses have increased mutual Opinion and may negotiate Alliances with one another.
Blood Brotherhood is available through Nomadic Government, the Way of the Nomad Trait, or via Tradition.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Blood Brother is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Blood Brother is a special form of Friend and Ally." It is part of Special Relation. The game also links it as Blood Brothers, Blood Brotherhood, Best Friend's and Best Friends'. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.