Why is it so inadvisable to have two people from the same family as your individual clients in psychotherapy? This person X comes along who is the mother/daughter/father/son/brother etc of your existing client A. Everything in your training tells you not to take on this new client X because it will contaminate your relationship with your existing client A. Of course, we are not rule-bound robots and situational ethics have a place but one is alert and wary!
This magic circle idea, or crucible is very important in my work. Yet at this moment I am thinking of breaking such a container (not exactly drasticall by doing therapy, but by having some familiy discussions to hold it.) Where does that fit in? In this context this item is worth looking at again.