Another Cat, Different Problem

Geeze.....they say things come in 3's...here's problem #2.

We have 6 cats....3 sets of sisters...all neutered. All adopted at about 2 months old.

oldest are 10 year old siamese (Taxicat and Shado)
middle girls are 6 yr old shorthairs (Gizzie Girl and Furby)
youngest are yr old longhairs (the twins-Bonnie and LouLou)

Gizzie Girl is the problem, we have nicknamed her BullyCat. It started about 2 months ago. She is stalking and cornering the twins at every chance she gets. She is unrelentless. Nothing playful about her actions. I have noticed that the twins now are becoming very very nervous, always looking around.

The twins were introduced into the household via the master bedroom, exchanging smells via revolving towels and paws under the door. Once the door finally opened they somewhat knew one another and the exchange of hissing only lasted about 48 hrs.
Then all seemed to enjoy each others company with no hostility.

We have racked out brains trying to think if anything changed around the house approx 2 months ago that might have upset her.
Have come up with nothing.

Have tried giving her some space......spending time alone in her favorite bedroom for a few days. Did not work.
Tried separating the twins into their own space. This did not work also.

Help!