x1 and x2 are mutual sibling nodes in a directed tree graph x6 such that their shared parent node(s) x5 is/are the most-recent common direct ancestor of x3 and x4, such that x1 is a direct ancestor of or simply is x3 (as the case may be), and such that x2 is a direct ancestor of or simply is x4 (as the case may be), all according to tree ordering relation x7.
x3 and x4 must be distinct nodes in the same directed tree; else this relationship is undefined. If x3 is a direct ancestor of x4, or vice-versa, then this relationship is – again – undefined. x6 and x7 define the relations/standards which are being applied. This word may be used in reference to non-familial tree graphs via analogy, despite references to 'siblings' etc. See also: "tunba", "dzena", "treicu", "tseingu", "kraune". At the time of creation, there was no commonly-recognized or succinct terminology, description, or phrase for this relationship in English to Curtis Franks' knowledge, other than something substantially similar to that which has been presented in the definition or gloss keywords.