Aggression towards other cat...

[badkat]

I have two female cats from the same litter that are 12 1/2 years old. Buttercup was always rather timid, Haley was the 'boss'. Ever since the loss of my third cat, Annie, in February, the Buttercup has been acting out, talking constantly, following me everywhere, and being a real pest. In the past couple of weeks, she has started stalking her sister. She sits and watches her until she growls and runs away, and then chases her. Haley has taken to hiding under the bed or the sofa all the time. Buttercup paces back and forth on the bed to keep her sister off at night. They are both very healthy, and neither was especially close to Annie. I swear I can see Buttercup laughing when she does this. I don't know what to do. Any help would be so greatly appreciated.