tamne gismu

x1 is cousin to x2 by bond/tie x3; [non-immediate family member, default same generation].

Probably preferred for metaphorical siblings (over bruna). See also dzena, famti, mensi, bruna, tunba.

In notes:

bruna
x1 is brother of/fraternal to x2 by bond/tie/standard/parent(s) x3; [not necess. biological].
dzena
x1 is an elder/ancestor of x2 by bond/tie/degree x3; x1's generation precedes x2's parents.
famti
x1 is an aunt/uncle of x2 by bond/tie x3; x1 is an associated member of x2's parent's generation.
mensi
x1 is a sister of/sororal to x2 by bond/tie/standard/parent(s) x3; [not necessarily biological].
tunba
x1 is a sibling of x2 by bond/tie/standard/parent(s) x3.