I'm not 100% sure but the way I understand it is there are 3 people. 2 of those people have one person in common. Like the relationship literally looks like a V with the common person being at the point of the V and the partners the arms. So the person in common has a relationship with both people but the "arms" don't have a relationship with each other. Make sense?
Sent from my iPhone...excuse any autocorrect mishaps!