So sorry to hear you're sister is having such a bad time. I'm starting to some out the other end of PND. The help of my family, especially my sister - if your sister is okay with the idea maybe and dad is what has kept me going. Part of the reason they were so great was the fact they (and some other freinds who have been really supportive since i was able to face seeing them( have had MH problems themselves in the past, and so they didn't try to talk me into cheering up, they just listened and provided practical support by visiting me, keeping me company, taking me out for a coffee, taking me up to the doctors when I couldn't make myself go (i.e. when initial dose on meds weren't working).
Perhaps if you have any freinds / family etc who have had problems and come out the otherside, they might be easier for her to talk to than a group therapy session?
In my case I also had a health visiting team that we great - they were outside the family so I didn't have to censor myself for fear of upsetting them. They came out on lots of home visits and provided lots of reassurance and help with self confidence as well as tips for baby massage etc that helped me find a way to keep going. Much more effective support than the nurse practitioner who has been supporting me at the GPs - nice enough, but interested in adjusting meds only and nothing else (or so it seemed when I was not well - my family tell me this was not really the case).
I was offered refferal to mental health team and which I really wish I had taken it up, but I had to make a phone call to speak to them, and I never managed it. I probably should have asked for help.
If your sister doesn't feel able to ask for more help, or just thinks there's no point it may be a wonderful thing if you can help her acess it (with her permission. of course)
i wish you all the best