Firstly, my condolences for your troubles in childhood.

That's true that people can learn psychopathic behavior through experience which is probably the case with your twin. People who are born as psychopaths are another type of predator entirely. With primary psychopathy, experience can shape the behavior, but the basic characteristics ... lack of empathy, lack of conscience, callousness, remorselessness, etc... is innate. In such cases, the brain tissue that allows most people to experience empathy and guilt is simply not present in the psychopathic brain.

On a brighter note, there's a good chance that your twin isn't a primary psychopath and, if she desires help, has a much better chance of healing than if she were a primary psychopath.