pentu experimental gismu

x1 is a non-binary sibling of/to x2 by bond/tie/standard/parent(s) x3; [not necessarily biological].

See discussion associated with "dinti" concerning whether this meaning is restricted to "affirmatively nonbinary" or whether it is more general. See also: "dinti", "nunmu", "tunba", "mensi", "bruna", "tamne", "famti".

In notes:

x1 is a non-binary offspring/child of parent(s) x2 [not necessarily biological]; x1 is filial (but not a son or daughter).