I went through a long period of what the docs called "pseudo-seizures." It was like grand-mal seizures without the epilepsy part. I didn't understand it at the time. I think I do now.

I repressed, stressed, tried to keep everything locked away inside, and it began to destroy me from the inside out. Sometimes it would just flat-out overwhelm me physically and neurologically.

If this is what is happening to your sister, it is not harmful, per se. Stay with her during the episodes, make sure she doesn't accidentally injure herself... especially her head. Talk to her, and tell her she is safe and needs to come back. Try massaging the back of her neck if you can.

Also, as she begins to come out of the episode, see if you can get her to slow her breathing, focus on your voice.

I hope this helps a little.