If I understand your requirement correctly, you can use:
[not(MyFirstModule.C_B/MyFirstModule.B)]
[MyFirstModule.C_B/MyFirstModule.B/MyFirstModule.B_A/MyFirstModule.A/MyFirstModule.B_A = '[%CurrentObject%]']
The first constraint filters out all C objects which are already associated with your current B object.
The second constraint shows only C objects with a path to an A object (through some B object), but that A object has to be the one associated with your current B object.
Hi Rom,
It is so simple that I don't understand why I did not find it myself.
It works fine!
Thanks