get
https://apibeta.familysearch.org/platform/tree/persons//relationships/
The Relationship Finder resource is used to find how two persons in the tree are related. For finding how the currently logged in user is related to a person in the tree, pass in "CURRENT" for the first person Id. The later option requires an authenticated session. To determine which of the persons returned is the common ancestor the fs:displayProperties element will contain a "role" field that contains "commonAncestor" as a value.