It's simple, it works like friends, except that you have to be friends before you apply for a relationship.
You have the choice between several types of relationship in a popup at the time you make the request.
So the button "add relation only appears on the profile of your friends.
Also the relation system must have been activated in the developer module.

ok, in the developer module? Haven't it installed yet, why is that? I will check, thanks for the hint