Why wait? It seems to me that boss's handling of this situation is long overdue. And if boss can't control her subordinates, then she doesn't need to be boss. Period. I think I would get together my list of grievances and go to her and tell her what's going on. Inform her that you are sick of this junior high garbage and you're sick of being made to feel like you have to lower yourself to PITA's level. She needs to handle it as PITA is out of control.
You've allowed this triangulation for too long frankly. I know there's really been precious little that you could do, but you need to seize this chance and get this thing solved for good and all.
I'd also request that I wasn't partnered with PITA further, siting personality conflict.