x1 is a sibling-in-law (sibling of a spouse) of x2 according to custom/law x3.
For "spouse of sibling" use "tubyspe", "bunspe" (brother's spouse), "me'ispe" (sister's spouse); for other "sibling of spouse" use "spebu'a" (brother of spouse) or "speme'i" (sister of spouse); for "sibling of one's sibling's spouse" use tubyspetunba vel sim. See also: spelanzu.