Just a quickie, this website is very helpful
http://www.nopanic.org.uk/
I used to use it at the height of my panic attacks (several years ago) when I was getting them 3/4 times a day.
They have a telephone number, which you can ring 24 hours a day, with a pre-recorded message, which talks you down.
I've recently started feeling "panicky" again, and have just discovered this other website
http://www.nomorepanic.co.uk/ which is very good, too.
They have a live forum, where you can talk to other people who know exactly how you are feeling. (I mean your sister, when I say that)
Do get her to visit her GP, as they will be able to refer her to someone who can help.
Cognitive Behavioural Therapy is often used for panic and anxiety problems, although there will probably be a waiting list if you go via your GP/NHS.
If it's really bad, and you don't think she can wait, have a look online or in the yellow pages, and find someone local to you who you can go through directly.
The longer she leaves it, the worse it will get.
I'd never really read up on panic attacks, until I got chatting to a fellow "sufferer". He emailed me the details of a website (above) and when I read the symptoms of panic attacks (racing heart, cold sweats, tight chest, sickness, pins and needles in the hands, trouble breathing) I realised I wasn't going mad after all, and these feelings were perfectly "normal" and quite common.
I felt very alone when I was suffering, until I did something about it.
The best thing you can do is be there for your sister. Make sure she knows that you're only ever at the end of a line, day or night.
The best bit of advise I was given was, nothing bad can happen to you.
I would sit there, concentrating on my heart beat, feeling for my pulse to see how fast it was beating, breaking out into a cold sweat, convinced I was about to have a heart attack.
Then the breathing would get more rapid, and I'd start feeling my hands tingling.
They would tingle so much, they'd eventually seize up, and almost lock.
I later found out, this was due to lack of oxygen.
Worst case scenario, I'd pass out. BUT... when I came round, everything would be back to normal. No fast heart beat, no rapid breathing. It was almost like a on/off switch, and would "reset" me.
If she does start hyperventilating, tell her to cup her hands over her mouth and just breath into her hands, for as long as she needs to. It may take a while, but it does help.
Definitely get her to the doctors, though. It won't go away, it'll just get worse.
It's really nice that you're asking on here. It shows how much you care.
Good luck :-) x