Hi Amanda! I think you need to pick and choose your battles wisely and I would not make an issue of this at this time. I've been in this situation too with my X and 5 step kids and believe me, you have a long road ahead and you have to tread very softly. So I will tell you in plain talk ok? Back off, way off. These kids lost their mother, been uprouted from their grandmother, and now have a new stepmother. Sure, the daughter is most likely reacting to the new situation and you need to roll with the punches, for now anyway.
It is what it is, they are who they are, messy or otherwise. The last thing I would do is give away anything of hers. You have no right to do that. Hands off. Its her stuff, not yours to do with what you please. Its her room and if she wants it to be a pig sty, so be it. She obviously is on an emotional roller coaster and the last thing on her mind is being neat to suit you or herself. In her mind, its a non-issue. You can however set some boundries such as no food in the room and don't leave dirty dishes in there for sanitary reasons.
You are not going to "break the bad habit" but you can work with the bad habit. You are a teacher, so use those tools and teach. Chances are her mother may have passed before she was able to teach her a lot of girly things.
Choose a day that you are both inside, no one else home, a lazy day, and simply ASK her if you could help sort out her stuff, always ASK. Ease into the conversation and take it from there. Get some Rubbermaid containers: one for give away and one for undecided. Offer to help sort and put things she is not using for now in storage so she will have more room. But be sure its her choice to do this project. Make it fun, crack a lot of jokes, be silly, laugh it up while you work, order a pizza, and put on some of her favorite music while you work. Suggest she decorate the room and have a paint party and do the room in her favorite colors and take her shopping for a new comforter and curtains, maybe something like that. If she is not up for it, then she's not, and back off until she is. Leave the containers in there and see what happens. But I truly wouldn't harp on this subject with her and let it go for a while. Everything is just way too new for you all at this point. Give yourselves time to adjust.
You know what? Michael's room was always a horrible mess. It terrified me! You couldn't find a place to walk without stepping on something. Unlike Daniel who is OCD about his room to the point he even has his hangers in his closet color coded, black for jeans, white for short sleeve shirts, tan for long sleeve shirts and colonges have to be lined up on the dresser according to size a certain way, and DVD's are in alphabetical order. I swear it gets to drive me up a wall. I learned at the end of the day, somethings in this world are just not worth worrying about.