Well gee, grounding and other things, just do not work with her.
What a tough nut to crack.
Maybe somewhere, even if she doesn't know herself, she is needing attention?
Is she overshadowed by her twin sister? Compared to her?
She wanting to be her own person and not the same, just because she is a twin? That she is separate?
Does she get treated the same or different than her twin?
Sometimes these small innuendos, really affects a kid... we may not see it, but they do... and in a magnified way.
What about you put her into child Therapy or Counseling?
Better to do these things NOW... before she gets older and maybe worse... or it escalates.
Has anyone just tried talking with her... not lecturing or scolding, but just seeing her thoughts/ideas/hopes/fears/ideals etc.? Is she accepted for who she is?
She seems of course, to have emotion based issues... thus she acts out this way.
Or she just doesn't care and is caught up in all the things other kids are doing.
What are her friends like? Their parents?
She seems out of control....
I don't have any answers.
But, I think putting her into some kind of Therapy or Counseling would be a good start.