I can’t make a diagnosis on the basis of a letter of course. But the behaviors you report are consistent with antisocial personality disorder (ASPD). Psychopath is not an official diagnostic category in the DSM-5, the Diagnostic and Statistical Manual of Mental Disorders that is used by mental health professionals. None the less, “psychopath” is commonly used as a term to describe people who have little empathy for others. ASPD is found in .2% – 3% of the population.
I’m struck by the fact that she’s been behaving so inappropriately towards others since she was very young. A person with ASPD has a long history of manipulating, exploiting, and hurting others. They don’t take responsibility for their hurtful actions and show little remorse or guilt when called to account. They have little regard for the rights of others. Often they try to shift the blame for the negative consequences of their behavior on someone else.
They can do quite well meeting people in a social situation, but they have difficulty maintaining friendships over time due to their thoughtlessness. Employment is often unstable due to their irresponsibility, impulsivity, and inability to work collaboratively with others. Family members who have had to repeatedly deal with their negative, even at times violent behavior, find them hard to deal with.
It is confusing to others that the same person who can be callous and thoughtless can also be quite charming. Your sister is probably often “forgiven” by others who don’t see her very often so haven’t been subjected to the pattern of abuse.
Unfortunately, people with APD aren’t likely to seek help from a mental health professional on their own. From their point of view, everything is someone else’s fault. They feel justified in conning and hurting others because they believe that everyone is out for themselves. They can’t acknowledge that their beliefs about others and their behavior is not usual or normal. For that reason, it probably won’t be helpful for you to suggest to your sister that she see a therapist.
Regardless of label, your challenge is to decide what you will accept as the terms of your relationship with your sister. You do not have to tolerate hurtful behavior. You are under no obligation to continue a visit if she is unkind. When provoked by her, do your best not to fall into the trap of being as unkind to her as she is to others. Simply calmly state that what she is saying is unacceptable and leave the situation as gracefully as you can. You will feel best about yourself if you can hang on to your own integrity in spite of your sister’s provocations.
I wish you well.