I think maybe a good chat and explaining your there for her and offering her a second chance, you don't expect her to move in and be a babysitter or a cleaner BUT at her age there are certain things she should be able to do for herself and to help you out a bit.

1. Stack dishes and tidy up after herself in the kitchen
2. Keep her room clean (i'm sure your girls do this and they are a lot younger)
3. Set a couple of BASIC chores for her to do - dishes, maybe sweep the floor or something.
4. No Phone at school, no phone in public (ie at friends houses or at dinner etc), 1hr games/fb time on phone at night (or more if you agree)

I know your her sister and not her mum but it's YOUR house, you have your own kids to look after and your doing her a favor taking her in.. she needs to recognise she is NOT a child and she has some responsibility too.
I think a good chat with her though on what you expect is a good place to start.