The 14 year old is lying to everyone!?

I'm a 3rd party here I have a step daughter who's 8. she has a big 1/2 sister who lives with her dad and step mom. The 1/2 sister is 14. My 8 year old adores her mocks her and idolize her. The 14 year old is lying allot. Telling her step mom one thing and her real mom another causing BIG TIME FIGHTS...almost knock down fights. she's paying them against each other for attention and to get the focus off of her being bad to them. Now she's not my kid or my husband kid, but she see's my step daughter all the time that i'm raising. I don't want my 8 year old to be like her, it disgust me how she plays her parents. She even told her step mom yes she flat out lies to her real mom. Both know that the 14 year old is tying to cause trouble, but then they still think she's always telling the truth when she's not. I don't want my 8 year old to be involved ( she already saw one fight cause of it). How do i make sure that she's not influenced to lye and cause trouble like her sister when she idolize her big sister so much. She's also trying to make my 8 year old boy crazy talking about kissing and how she has to to it and stuff like that, when i don't think it's her position especially when she's so promiscuous. And I'm afraid if i say something to the 14 year old she'll drag us into her web of lies and get her real mom, also the 8 years real mom in a fight with us.