Hi, I'd like to help you with your question.
Sociopathy, clinically known as Antisocial Personality Disorder, usually starts early in childhood. There are some specific signs that mental health professionals look for including bed wetting, cruelty to animals and setting fires. These markers when seen together usually indicate an anti social personality. Additionally, the child will defy authority and fight often with others. They will also refuse to follow the rules.
As adults, people with antisocial personality disorder will often come across as narcissistic, charming, and will act with violence when confronted. They have little difficulty with hurting others. They will lie and cheat and rarely have very close relationships.
It is important that you try not to fix your sister. There is no way to stop her behavior.The best way to handle her is to try to protect yourself the best way you can. Start by educating yourself and finding support. Here are some resources for you:
The Sociopath Next Door by Martha Stout
Without Conscience: The Disturbing World of the Psychopaths Among Us by Robert D. Hare
Difficult Personalities: A Practical Guide to Managing the Hurtful Behavior of Others (and Maybe Your Own) by Helen McGrath PhD and Hazel Edwards MEd
You can find these books on Amazon.com or your local library may have them for you.
Try to keep your distance as much as possible from your sister. Minimize your contact and your family's as well. It is hard since she is your sister, but the more you can protect yourself, the better.
I hope this has helped you,