My 15yr old sister has moved in and i need HELP

[Fleur]

Ahh teenagers. Fun much?

Good on you for giving her a place

As ZF said, it's a good idea to sit down & talk with her. I'm not sure how receptive she is, but try explaining how it makes you feel when she leaves stuff lying around etc. Try not to go overboard...pick a few of the WORST things, or keep it simple.

My teenage DD hates it when I make statements like 'you always...' (even when it's true, lol) so I try to keep it as accurate as possible - 'I feel like you do X a lot, and it frustrates me becuase I am trying to keep things tidy'
and that helps keep her form becoming too defensive. If she feels like I'm attacking her she just closes up & nothing I say gets through.

If you can, offer her a trade - you expect A,B,C and she can have / get / do X,Y,Z. It doesn't have to be monetary - it might be as basic as, if she picks up her washing then you will do it with the rest of the family's washing. If she leaves it lying around, then she has to look after it.
Get her to enter into an agreement with you on it, and get her to verbalise it - 'I will....' You might have to do the same. My teenagers both are really big on fairness. You cannot ask for more than you expect.

Most of all be patient, it will take time for things to change. She's following you around all day... she sounds lost Give her firm boundaries & a loving safe place, and then time...
Good luck

ETA - just read your last post - consequences are great things. Limiting their wardrobe if they leave clothes lying around is a really appropriate response I think. DS1 has just lost his iPod charger because he continually leaves it hanging out of the computer...he was warned, now it's gone for a week.