To relate two clients to each other you can go to Clients -> Clients.

Once you see the client's list, click on  in the client you want to relate to others:

Now, go to the Additional Information section then click on the tab Related People:

Click on  NEW to add new relationships between clients. Once you've clicked you'll see a window like this one:

Here you can look for the person related to the client, and also, you can state what kind of relationship they have. When you're done press Update.