Hello, do you know how can i implement the relationship between users?
User A is the Father of User B
User B is the 1st Child of user A
User C is the Younger sister of User B
User B is the Older Brother of User C
User E is the husband of User A
User A is the wife of User E
The extension MB Relationships just helps you to create a relation between objects (post/terms/users). In your specific case, you need to create many codes to exclude users from a select box if they are already selected in another box like a family.
Using the setting query_args will help you to do that https://docs.metabox.io/extensions/mb-relationships/