My daughter has had night terrors before...but, as I understand from our doctor and from reading about them, night terrors are something that a child doesn't remember. My daughter certainly did not remember them at all. She was asleep during the whole situation. My husband and I on the other hand remember everything and just were horrified having to deal with them.
Nightmares, on the other hand, a child will perhaps remember because they actually can wake from them, and could potentially tell you about them.
The only thing that can be done from what we've read and have been told is to work thru them. Assure the child that nothing can happen to them...that you're there to protect them. If they truly remember what scared them, work together on something that they can "defend" themselves with. I remember a friend of mine worked with her daughter on a "art project" which ended up being a shield to put above her door to ward off the boogie monster. And her daughter loved it. And then she was able to pick out a stuffed animal at the toy store that she slept with that would make sure she was safe. And it worked.
With my daughter, since she didn't remember anything of the events, we just had to hold her and console her thru the ordeal...it usually took her about 30 minutes to an hour to calm down and lay back down. She was out immediately after putting her head on the pillow. It was absolutely horrible. And I really hope she doesn't do that again any time in the near future, but there's really nothing that can be done expect to talk calmly and soothingly during the episode, hug them, turn on all the lights and carry them around if they are willing so they can "see" that everything is fine, and continue to try and calm and talk with her. Assure her that you're there to protect her and that nothing can harm her. My daughter hasn't had an episode since earlier this year.
Sorry there isn't anything else I can offer. A feel for your sister! It's terrible and heart-wrenching. But they will get thru this with lots of encouragement and creativity. Ask her if she wants to sleep with the lights on too...my daughter liked that.
Best of luck to your sister!
Cheers, C.