speselrirni lujvo

x1 is the step-child (child/offspring of the spouse) of x2 by law/custom/standard x3.

See: "(se) rirspe", which is largely equivalent.

In notes:

x1 is a daughter-in-law/son-in-law (a spouse of a child of) of x2 according to law/custom x3.
x1 is the spouse of the child/offspring/protegé of x2 by standard/law/custom x3.
x1 is the parent-/mother-/father-in-law (parent of spouse) of x2 according to custom/law x3.