Blood Siblings

For notes on the relationship akin to human siblings, see Litter Mates Since the earliest pre-Unification conflicts, Stenza have recognized the bond that the fires of war forge between people. Peculiarities in Stenza psychology mean that this bond most commonly forms between pairs, although trios are not unheard of. One traditionally refers to one's counterpart in this relationship as their "swlê". Pre-Unification, such bonds formed mostly between members of the same clan and occasionally with members of an allied clan in any given conflict. However, the greatest revolution to the concept came during the Battle with the Stone Menace, an existential conflict between an unknown creature and the entire Stenza species. By the end of the war, all Stenza clans had been fighting and dying alongside one another, protecting each other's children and securing each other's food. This vast web of bound relationships was unprecedented, and ultimately one of the greatest contributing factors to the initiation of lasting inter-clan peace. This relationship is considered to be completely governed by the god Iradae, from its inception until both parties are dead. Therefore, each party is considered to be an involuntary participant (in contrast to courtship). This also means that it is impossible to intentionally become blood siblings with another person, as the initial stages where the pact is forged are held to be driven entirely by a divine third party. The social and legal quirks of this type of relationship include but are not limited to:

a number of contexts in which each party to the relationship are held to be the same person (including during trial, when one member cannot testify against the other, as it would be akin to testifying against yourself)
blood siblings serve as mediators in each other's love lives (including playing wingman and warning paramours when they're getting a little too attached a little too early)
belonging to the same clan in certain cases (this is often a factor in marriage negotiations)
serving as a "vessel" of each other's social capital (most famously, Ta'zhen had received a lot of clout for being known as the swlê to then-well-regarded First Officer Tzim-Sha) or as someone who can vouch for or protect and shelter the other in certain social situations

The bond is considered to be unbreakable, although it's worth noting that virtually no one has ever thought to try (and those who do tend to keep that to themselves). The web of blood relationships is considered to be the cornerstone of peaceful Stenza coexistence and so anyone who "breaks rank" so to speak risks pulling on the thread and causing other people to fall away as well, leading to a collapse back into pre-Unification civil war. Cooler heads believe inertia will at least carry for a generation or two, however.