Well, it appears my foot's firmly planted in my mouth.
It's not crossing any ethical guidelines, however, the situation obviously makes this more complicated than other cases. If you've not told your therapist about this yet, I would recommend bringing it up with her, as these feelings manifesting with her, rather than your mother, could be useful in dealing with some of your problems by proxy without any real risk. If you have told her about it and she's done nothing, I would recommend getting a new therapist - like any other kind of relationship, sometimes a therapeutic relationship just doesn't work out.
Are you undergoing any specific PTSD treatment (whether psycho- or pharmacological) for this, or are you receiving psychotherapeutic counseling?