I wonder how much insight she has? You say she is 13F. Most girls are that age, if not a bit earlier are becoming very aware of their bodies, and usually tend to be conscious of their living spaces... granted this is just a generalization, not a definite.
When you have spoken to her about this, what do you talk about? Do you know why she is like this? Is it laziness due to being spoiled? Is it that she isn't aware of why people need to keep their living space clean? Many kids her age just think of these things as tasks that they're now starting to have to do because they're getting older. Is she simply being rebellious because she doesn't want to take on responsibilities?
These are all important things to talk about if you want to find an actual solution to fit the issue. If she's think's that tidying and cleaning is mundane, explain to her - use visuals, iPads work great - show her the nastiness that caking up in her room. Most young kids respond pretty well to that type of thing in my experience.
If she's simply lazy, that is something is going to take consistent but constructive prompting. Often it helps to have a rule in place prior to the actual clean up time - by that I mean, mention for the 2-3 days prior that on Xday everyone's going to clean their rooms before they can do any socializing, cell phones, friends, going out - all gets done after rooms are clean.
I actually helped design a program specifically dealing with youth who resist completing activities of daily living. This approach has worked very well in group homes, especially with females around your sister's age.
Taking into consideration your and your parents schedules: Regularly plan a "major room/major chore" day (full clean of the room - shouldn't really take more than an hour or two at most - even if it does, it's okay as this actually helps reinforce keeping it clean throughout the week)
Also regularly schedule a fun activity for that day, something that she likes to do. Whether it's a family activity, or getting to go to the mall for a couple hours or the movies, whatever, make sure she's got something to look forward to later on. Yes, this is leverage, but it tends to work well with resistant kids.
When you present this to the misbehaving child, it's important to do it in a way that does not appear as though you're coercing them into doing something they don't want to do.
My suggestion is to approach the conversation like this: "Hey Anne, can we talk for a few minutes as a family?" Then, each of you calmly tell her your concerns without conveying anger or making her feel ashamed. This doesn't have be drawn out into an intervention-style meeting. What needs to be clear is that this is how it will be from now on, until the bedroom is maintained in a reasonable manner.
You want to ask her how she feels about her room (after you've given your concerns she may start to have a few of her own that she hadn't thought of) if so - explore those concerns with her as much as you can, don't down play them or tell her not to worry about them, just encourage her that it can be fixed and you're all there to help.
It may be difficult but the key here is to have her really buy into the idea that her room isn't safe or healthy in ways that she may not even be considering. She may face serious bullying if her friends talk about what they see.
But first, start with the "why" - that's the most important thing. If she's having mental health problems that could be part of it. Depressed people often let their homes get atrocious for periods of time.
Once you've got her on board at least with the idea that her room isn't safe, is when you need to be firm with her and lay out the ground rules. For our kids it was Saturday is cleaning day - if you wanted to go anywhere on Saturday or Sunday, your room and chore had to be done and approved by one of the house workers.
I got weekly reports for 3 months, and by the third month, 18/20 kids who were in the program were keeping their rooms clean throughout the week, and completing their major room clean and chore without having to be prompted by staff. The two outliers were sisters who happened to be drug users, often they were simply unable to complete their room and chore, and didn't care that they wouldn't be able to go out.
Note: I know this is done by parents all over the globe, I know I didn't invent this method of motivating kids to do things, I just wanted to throw it out for OP as I have a lot of experience working with kids who sound a bit like his sister.
Hope that helps, sorry for the long post :/