Now I don't want to come across as the wicked step sister but I have reason to believe that my younger step sister is a sociopath. I haven't said anything about this to anyone as I am in no position to diagnos her since Im not qualified, however I can't find any other explanation for her behaviour. She is unable to show appropriate emotion, allienates her self from the family, and lies about everything.. even the small thing like what she had for breakfast or did she clean her teeth are lied about. Our family is rather complex as it is really two families joined together she is my step sister so I don't feel able to tell her father about her as I know that saying something against a parents child is a very hurtful thing to do and I don't want him to feel as though I'm trying to score points. I have spoken to my mother and she agrees with me that there is a strong possibility that harriet (not real name) is a sociopath. My question to you is should I express my beliefes to the father or keep quiet because I am clearly over reacting.. keeping in mind that for some reason the father has a tendency to be very protective to harriet more so than his other two children. I really don't want to hurt anyone but her actions are affecting all the children in this family in a negative way including the parents.
Thank you

Does she act out in school? If this is the case maybe, you could get her teacher or a counselor to call him or have him to come in for a talk. It sounds like he knows something is wrong. Good luck

How old is she? Besides the lying and alienation what other behaviours does she display? It could be simply she has emotional issues from the changes in her life or an attatchment disorder but here is some info on Sociopaths, if this fits "Harriet" I would find a suitable time to speak to her father and clearly explain your concerns for "Harriet" and that it is not a attack on his daughter.
Glibness and Superficial Charm


Manipulative and Conning
They never recognize the rights of others and see their self-serving behaviors as permissible. They appear to be charming, yet are covertly hostile and domineering, seeing their victim as merely an instrument to be used. They may dominate and humiliate their victims.


Grandiose Sense of Self
Feels entitled to certain things as "their right."


Pathological Lying
Has no problem lying coolly and easily and it is almost impossible for them to be truthful on a consistent basis. Can create, and get caught up in, a complex belief about their own powers and abilities. Extremely convincing and even able to pass lie detector tests.


Lack of Remorse, Shame or Guilt
A deep seated rage, which is split off and repressed, is at their core. Does not see others around them as people, but only as targets and opportunities. Instead of friends, they have victims and accomplices who end up as victims. The end always justifies the means and they let nothing stand in their way.


Shallow Emotions
When they show what seems to be warmth, joy, love and compassion it is more feigned than experienced and serves an ulterior motive. Outraged by insignificant matters, yet remaining unmoved and cold by what would upset a normal person. Since they are not genuine, neither are their promises.


Incapacity for Love


Need for Stimulation
Living on the edge. Verbal outbursts and physical punishments are normal. Promiscuity and gambling are common.


Callousness/Lack of Empathy
Unable to empathize with the pain of their victims, having only contempt for others' feelings of distress and readily taking advantage of them.


Poor Behavioral Controls/Impulsive Nature
Rage and abuse, alternating with small expressions of love and approval produce an addictive cycle for abuser and abused, as well as creating hopelessness in the victim. Believe they are all-powerful, all-knowing, entitled to every wish, no sense of personal boundaries, no concern for their impact on others.


Early Behavior Problems/Juvenile Delinquency
Usually has a history of behavioral and academic difficulties, yet "gets by" by conning others. Problems in making and keeping friends; aberrant behaviors such as cruelty to people or animals, stealing, etc.


Irresponsibility/Unreliability
Not concerned about wrecking others' lives and dreams. Oblivious or indifferent to the devastation they cause. Does not accept blame themselves, but blames others, even for acts they obviously committed.


Promiscuous Sexual Behavior/Infidelity
Promiscuity, child sexual abuse, rape and sexual acting out of all sorts.


Lack of Realistic Life Plan/Parasitic Lifestyle
Tends to move around a lot or makes all encompassing promises for the future, poor work ethic but exploits others effectively.


Criminal or Entrepreneurial Versatility
Changes their image as needed to avoid prosecution. Changes life story readily.

Contemptuous of those who seek to understand them
Does not perceive that anything is wrong with them
Authoritarian
Secretive
Paranoid
Only rarely in difficulty with the law, but seeks out situations where their tyrannical behavior will be tolerated, condoned, or admired
Conventional appearance
Goal of enslavement of their victim(s)
Exercises despotic control over every aspect of the victim's life
Has an emotional need to justify their crimes and therefore needs their victim's affirmation (respect, gratitude and love)
Ultimate goal is the creation of a willing victim
Incapable of real human attachment to another
Unable to feel remorse or guilt
Extreme narcissism and grandiose
May state readily that their goal is to rule the world

Thank you both for your advice, to answer your first question bushg, yes she does act out in school my parents have had countless phone calls home about her bullying the other children. For example the worst one was when she sent horrible text messages to a girl in her class from that girls friends telephone after she stole it.. further more our parents were called into the school by the head master and the victims mother... while our parents were in the meeting harriet cornered the victim and I don't know what she said but the victim was crying so much that her mother had to take her home from school. I don't actually know much about the faculty at her school and wouldn't know about how to get in touch with a member of staff there and I don't feel that I am in a position to contact them as I'm sure her mother and father would not appreciate it. But thank you for your suggesstion.
MichaParker, my sister is 11 and I know from research that you can't officially diagnos one as a sociopath until they are 18. However thank you for your information, she certainly fits the large majority of traits on this list, especially harming people (who are smaller than her) take for example my baby half sister... it is come to the point were we cannot leave her alone with her because the baby always ends up screaming as she will either pinch, hit or push her over but then pick her up and comfort her which I cannot understand why? There does seem to be an obsession with sex also as we found porn 3 time on her computer (which she is no longer allowed on) and once on her mobile phone. I wish I could tell her father but I know it will only cause chaos he seems in denial of her and lets her off with more than the other children saying that its because she the 'youngest' and just a baby... which is true if you only include her immediate family where her sister is 13 months older than her but not if you include her half sister who is 9 years younger than her. Thank you for listening and for all your advice I'll try to put some of it to practice but like I said I don't feel as I'm in a position to do so... any other suggestions?
Thanks again.