x1 is a relative of x2 such that they both share an effective family tree which includes a common relative x3; x1 is a relative of x2 on x3's side of the family.
x1 and x2 are technically mutually symmetric under exchange. Technically, x1 (therefore, also x2) and x3 are mutually so, as well. However, x3 will typically be used in order to identify the 'side of the family tree/genealogy' being considered and x2 will typically be the focal point at which the several sides of family trees (including the considered one) intersect (being otherwise disjoint); as such, x2's paternal uncle will not be described as being x2's father's brother on x2's side of the tree. In other words, there is a large family tree with at least two 'sides' and they intersect at exactly one node; this node of intersection is canonically to typically be x2 and x3 is canonically to typically be used to designate which of these possible sides is being considered such that x1 belongs to it. (Symmetry is enabled by the fact that the only inferrable family tree is the 'side' produced; the larger family tree is unknown, and so the collection of potential sides amongst which the specification is taken is also unknown). The largest family tree (graph union of all sides of the mentioned relatives) is the graph which the utterer and/or relatives mentioned consider to be the entirety of the family of all the relatives mentioned (plus some near relatives, so long as they belong to exactly one side); thus, it may include marriages, god-children, close friends, and exes, but it is unlikely to span over many generations. For example, maternal and paternal sides of families are considered to be separate sides of the family tree for a child, even if in the distant unknown past, that father and that mother share some sort of familial relationship (even including a biological common ancestor). This word can be used to hint at a path in the graph from x1 to x2, since there exists a path which connects them and which also includes x3; therefore, it can be used to vaguely mean "my onclic aunt-in-law by my Uncle John" meaning that the referent is the utterer's Uncle John's wife's sister (or sister in law); use connectives in x3 in order to include (if the connective means "AND") more nodes which must have paths (not necessarily the same ones) running through them from x1 to x2. This word does not actually specify any of the relations between x1, x2, or x3 arguments; it merely claims that they are all related and belong to a common (effective) family tree. Use tanru or lanzu for greater detail.