To relate two clients to each other we must first access:
Customers -> Customers.
Once we have entered the list of clients we will press the icon 
of the client that we want to relate to others:
Now, we will access the Additional Information section of the Customer Profile and within the section, we will go into the Related Persons tab:
As we see, there are two sections, one is that of family unity and the other is that of related persons.
In this case, we are interested in pressing the icon 
in the related people section, to add new relationships between users.
Here we can look for the person related to the client in question, and we can also indicate what kind of relationship they have between them.