tunba -tub- gismu

x1 is a sibling of x2 by bond/tie/standard/parent(s) x3.

See also bruna, mensi, tamne.

In notes:

bruna
x1 is brother of/fraternal to x2 by bond/tie/standard/parent(s) x3; [not necess. biological].
mensi
x1 is a sister of/sororal to x2 by bond/tie/standard/parent(s) x3; [not necessarily biological].
tamne
x1 is cousin to x2 by bond/tie x3; [non-immediate family member, default same generation].
selfamti
x1 is an niece/nephew of x2 by bond/tie x3.
paznu
x1 is a non-binary offspring/child of parent(s) x2 [not necessarily biological]; x1 is filial (but not a son or daughter).
pentu
x1 is a non-binary sibling of/to x2 by bond/tie/standard/parent(s) x3; [not necessarily biological].
tcuti
x1 is a sibling of a direct ancestor/elder of x2 by bond/tie/relation/standard x3 (not necessarily genetic/biological nor necessarily directly ancestral); x1 is an associated member of one of x2's ancestor's generation; x1 is an (great-(great-(...)))uncle/aunt of x2.
dzu'enba
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.