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.
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.