S., you have already discovered isolation and groundings are not working. Sounds like it's time to dig deeper into the problem... assuming there is one. It may just be that one daughter is simply of a different personality than the other and this is just the way she will be, to some extent, for the rest of her life. On the other hand, there may be some underlying issues at play concerning her anger and hurt (and that is more likely what it is and not just plain outright meanness - only a child with an emotional disturbance is likely to be mean mean at that age as opposed to merely manifesting anger and hurt over something. But, if the anger is not addressed when they are young, it will become meanness when they get older). Has she always been this way -- since infancy? Either it is her character or she may have a personality disorder.
Has the attitude come on more recently? Say, in the last two or three years? Again, it could be the result of nothing more ominous than her personal character developing; or it could be a relationship issue. This is the most likely scenario. One of your twins is naturally the dominate personality. If the temperamental one is the 'second twin' she may be developing feelings of inadequacy around her sister ... this most especially if the temperament has developed just since the girls have reached school age. Are they in the same class at school? If so, be sure to stress to their teacher to refrain from comparing the girls. They are seperate idividuals and, as much as they may be bonded to each other in many ways, they are still different entities and the school needs to retain a keen awareness of that.
Since the problem seems to be focused more on her older brother, is it possible he has discovered ways to torment her without you knowing? Siblings are very insightful concerning one another. He may have discerned her weak spot long before you and has learned how to take advantage of it. (You might even try to enlist him in your efforts to calm his sister but, don't count on it. Even if kids fight almost constantly, when the chips are down, they will form an amazing solidarity against any and all outsiders - and that includes the 'rents.)
Obviously, you need to talk to your daughter about her behavior. When you do this, it is important to remain calm and non-judgmental. It is not necessary to schedule a formal talk with her but take advantage of any openings in conversation - preferably not after she has had another display of the grumps but, if that is a viable time to broach the subject ... go for it!
("You know it really hurts your brother's feelings when you treat him like that." (Don't be surprised if she comes back with accusations of him hurting her feelings. That is an ideal opportunity to sit down with her and pursue a talk.) "Your brother hurts your feelings? Is that why you seem so angry all the time? Well you know big brothers do that. Sometimes I think that's just what they do for fun. It's not very fun for you, though. Is it? Would you like me to talk to him about it?"
Approaching the problem from this perspective validates whatever feelings she may be having. It also serves to let her know you are concerned with her feelings and opinions and want to help her (NOTE: she is the one who has to resolve her problems; you are just there to help and guide her.)
As far as her other associated issue, the way she now speaks to others, as long as you are constantly reinforcing your love and concern for her in other ways, simply disregard what she says when she does not speak respectfully, and instruct others to do likewise. Explain to her she must be courteous and respectful when she speaks to others and, if she does not, you simply will not respond to her. In the beginning you might consider offering better phrasing for her comments and insist she use it - if only to instruct her in more desireable options. Later, nudge with a reminder to "use your polite words" or "what happened to your respect?" or just a simple, "Courtesy." Eventually, you will reach the point where you will not respond in any way unless she does use desireable language.
As with all your children, you do want to teach her the value of being courteous in future social and business situations, but that is a long-term process that will come one step at a time, and on a different schedule, for each of them.
Good luck.