Anxiety in all it's different forms can take over a person's life. Personal Counselling may help to demistify it's hold upon a person by initially explaining the physiological impact it can have upon a person, right down to emotional and behavioural implications. By working in therapy on addressing one's feelings and working together on agreed goals of therapy, this maybe a positive way forward to make changes in dealing with anxiety, general or otherwise.
It is perfectly normal to have a little anxiety in our lives to motivate us into acheiving things on a day to day basis i.e getting to work on time and so forth. But when it overwhelms or stops us from doing the things we want to do, or our lives seem out of control because of it, this may be a useful time to consider talking to a Counsellor.
Combined with PND, feelings of 'feeling dis-empowered' and life feeling 'out of control' maybe present. Seeking a Counsellor who is empathetic and is experienced in working with clients with these issues is the key, however a person has to want to go into Counselling themselves.
It's important to note not every Counsellor - client fit works. If when talking to your sister she expresses that she is not finding Counselling helpful, then a change of Counsellor maybe and option if she so chooses.