I would suggest a behavioral therapist (sounds like you are on your way there) and reading the book, the Explosive Child. That helped us early on. It turned out that our daughter has anxiety and ADD. Once she got treatment for both, she's a very happy kid. I tried everything before the diagnosis, diet, sleep apnea surgery, therapy, reading parenting books thinking it was us. It's not parenting, by the way. :) I always knew that deep down, but you get so much advice about how you need to do this and that and not let them "rule the roost". Yes, but these aren't neuro-typical children and they don't respond to typical parenting styles at all, no matter what the style or method used. We do see a small correlation between food dye and behavior too, btw. But, the main progress we saw was when she finally got help and medication for the ADD. She describes that she is happier, her thoughts aren't jumbled up anymore, she can learn quickly. It's amazing to see her come home and willingly do her homework now, compared to the fights we had for years. ADHD isn't just about focus, for my daughter it also caused severe emotional outbursts. The anxiety also doesn't help with frustration and behavior as well, since she is so "good" outside the home and then is comfortable enough at home to really unleash.
I highly recommend behavioral therapy and major empathy on your part. Less discipline and negative comments, more empathy and understanding really helps my girl when she's frustrated. Any time we get even slightly worked up, she feels it. She's very sensitive and we have to make sure we keep everything calm or she can sense the stress.
The other thing to mention is that you should sit down with your other daughter and explain that this is not her fault and that her sister is not acting the way she wants to act. I had to explain to the doctors that I was very concerned for my son, and feel that they finally took that seriously. I don't think they completely understood the toll it was taking on the family and especially her sibling.
Good luck. You are doing the right thing by getting her help. For now, keep in mind that no child wants to act that way. I always have that in the back of my head when things get stressful.