How long has she been with you? I would err on the side of caution and go gently with her. So perhaps just gently chat to her about the room needing to be accessed and give her some easy options to plonk stuff on/behind.
Moving out of home cos you have no options there anymore would be rough no matter how your behaviour has prompted it so I think she is probably after some reassurance that she has a place with you (and probably still with your mum etc). I realise you need to have things function but it may take some time to get your groove going together too. Is there any chance of tapping into some counselling for her (and you) to give you both some time to debrief and discuss your strategies with someone neutral? You both will need some neutral support so you can talk with out filtering your concerns/needs. I know it may sound OTT but she is still very young and you are her sister so renegotiating those boundaries will be difficult.
Does school have any options to support her? It is a tricky age to establish yourself again especially if you are feeling a bit lost anyway.
She is lucky to have you.