How do you tell the difference

Hi, I was just wondering how to tell the difference between someone who has a mental disorder with lying or just lies for all the moral wrong and negative reasons people usually do (i.e selfishness, guilt, etc etc)

Basically my sister is 13 now and she compulsively lies to the extent I dont know what to do about it, because she isnt going to get nowhere in life... i mean her lies aren't even convincing half the time !

She lies and steal, but not just like how you would expects a 13 year old girl to, even for things that arent important or things that she already been caught out for, she will STILL refuse to admit to.
For example she stole my hat, let a girl wear it at her drama play (which she invited me to come and see), instead of confronting her i was like, 'wow cant believe that girls got the same hat as me such a coincidence, why dont you go and take it from the drama club cupboard no one will notice!

For some reason she must of felt guilty she got the hat hid it in her bag or something and then when we got home, she got it out, but she had ripped off the tag and cut in inside a little bit... ' ye its a big damaged, its isnt exactly like yours' she explained to me....

another example she snunk into my room (god knows how because i have a chain lock on there) I came home and instantly knew, i was so angry, i just said ADMIT THE TRUTH just tell me HOW you got in there, and not matter how loud i screamed she just denied it, even though she was already getting punished for it...

This went on for about 2 years until social services got involved and ended up taking her to a foster family (mainly due the stigmatization of my mother who has a mental health disorder unrelated) Anyway I hardly see her now, and when i do she never talks about all the lies and stealing that went on at home, never. And her foster parents who make out shes an angel say she NEVER lies or steals from them, and I am HIGHLY doubtful of that, because she still lies to me about things when i have contact with her.

I just wish i understood why she did it, and why she can never tell the truth, she would let me other younger sister get in massive trouble, for lies she told ...

Hello delldell,

See, her foster parents are talking about her lies because they think she is so young now and as time passes, she will get teenager and will probably leave compulsive. I know, what you are talking here. You are worried for her lies. May happen, she continues lying. Who knows what will happen in future? So, here you should think about something, how you can tell her this is not good thing.

If she had stolen something of you, then leave that thing. Leave that thing with her, tell her this is yours. Give her love as elder brother/sister. Don't mess with her anymore. Keep saying her slowly, i know you, and i know you will not lie for next time. Try this for a few months and see what happens. Don't yell or scream on her. It will increase her lying. Yes, this is possibility. So be careful with her.