Hi CherryC,
I was on Mirtazapine for 6 months only and had horrible withdrawal symptoms that caused anxiety and panic attacks. Afterwards I changed my lifestyle completely and with help of supplements, meditation, CBT and lots of reading I started feeling better. I read later about Mirtazapine and would not recommend it, but that's only my experience. Occasional Diazepam helped with anxiety was at it's peak, Propanol didn't do to much for me, but I only took it occasionally what seems not to be enough.
I am sure that Mirtazapine and other AD can help some, but for me, after my experience, I decided never to go back. Although it was very hard to go through withdrawal, the thought of doing it again was worse than anything for me.
If I were you, I would read a bit more about specific AD's and withdrawal and make a decision, I understand your sister's worries. Possibly addition of testosterone to her HRT could help, there are quite a few treads here about this. x