some help with my sister please if you can

[fostermumtomany]

as you are all parents of teens maybe you can help me with my sister who is 14.
she was adopted by my parents at the age of 9 months.
recently she has turned into satan. my dad is disabled and she has been pushing him down the stairs. she lies constantly, steals from mum and dad, stays out all night, says she is going somewhere then goes somewhere different, climbs out her bedroom window when grounded, the list goes on. the most worrying thing though is that she has been seeing this boy who is 17.
well after a bit of digging i found out she is related to him via her birth parents.
they are first cousins. she knows who he is, she looked for her family through the internet and found him. he also knows they are related. they are in a sexual relationship. i havent told mum and dad as it would destroy them.
i have tried talking to her big sis to little sis but everything i say falls on deaf ears.
what can i do? she has been speaking to the school counseller but given the amount of lies she tells i am wondering what she has been telling her.
i know she told her best friends mum that my parents physically abuse her which is totally untrue, she is the one abusing them (pushing dad down the stairs!)

on friday she rang me at 9.30 pm to say dad wasnt well. she said he was having chest pains and pain in his arm. so i told her to go get mum and get her to have a look in on my dad. she told me she had sent dad to bed "cos he looks well ill"
anyway it turned out he just had indigestion. i rang him the next morning to see how he was and he said im fine why did you ring?
i told dad that she had phoned me and that he had chest pain and pain in his arm. he said he didnt have pain in his arm. when he said to her (while i was still on the phone) "why did you tell your sister i had pain in my arm" she denied it. then she said that i had phoned her worrying about dad!
after that dad went a bit off with me over the phone. i rang again today to tell them something about my little girl and 'whoever' answered the phone didnt say anything then hung up. so i rang back thinking it must have been a bad line and the same happened again!
why is she being like this i have never been anything but kind and loving to her i have always stuck up for her, always taken her side always tried to be a good big sister, now i feel like she has slapped me in the face and i really cannot stand to see her treating my family in this way. what do i do?

[FetchTheMaid]

Oh dear - your last line says a lot - "her treating my family in this way". It's her family too and maybe you see her as an outsider and that is how she feels?

Don't how you would resolve this. Hopefully someone with experience of kids having similar situation/going to counselling will be able to help.

Good luck

[kreecherlivesupstairs]

IWY, I would repost this somewhere with more traffic.
I do know that 14 is not old enough for sex. Age of consent is 16, although that's the least of your problems.
Your sister sounds like a real little charmer.