sperirni lujvo

x1 is the parent-/mother-/father-in-law (parent of spouse) of x2 according to custom/law x3.

See also: rirspe (spouse of parent), spemamta, spepa'u, selrirspe, speselrirni, be'aspe, spebe'a, ti'uspe, speti'u, spelanzu.

In notes:

rirspe
x1 is the step-parent/step-mother/step-father of x2 due to marriage with their parent according to law/custom x3.
selrirspe
x1 is the spouse of the child/offspring/protegé of x2 by standard/law/custom x3.
spelanzu
x1 is the family of the spouse of x2, composed of members x3 via bonds/connections x4, where the spousal arrangement is in accord with law/custom x5.
spemamta
x1 is the mother of a spouse (mother-in-law) of x2, according to law/custom x3.
spepa'u
x1 is the father-in-law (father of a spouse) of x2 according to custom/law x3.