mensi -mes-me'i- gismu

x1 is a sister of/sororal to x2 by bond/tie/standard/parent(s) x3; [not necessarily biological].

See also bruna, tunba, tamne, famti.

In notes:

bruna
x1 is brother of/fraternal to x2 by bond/tie/standard/parent(s) x3; [not necess. biological].
famti
x1 is an aunt/uncle of x2 by bond/tie x3; x1 is an associated member of x2's parent's generation.
tamne
x1 is cousin to x2 by bond/tie x3; [non-immediate family member, default same generation].
tunba
x1 is a sibling of x2 by bond/tie/standard/parent(s) 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].
mes
Sister