I share a bedroom with my younger sister, she is currently 15 and I am 20. Recently I have been becoming very stressed out with the clutter in the room. I begun cleaning out my own belongings, following the KonMari method of disposing of things that no longer spark joy. This is going really well - I have removed a lot of my own clutter and am still working to improve it.
However, it's quickly becoming obvious that my sister is a large cause of a lot of the clutter, so much so that she's metaphorically shooting herself in the foot with the sheer quantity of unused and unwanted things that she keeps around.
One example of this is our desk. On the desk, there is a pile of her old school books, and also a pile of her clothes that she doesn't wear or no longer fits. Mixed in with these clothes is occasionally pieces of her school uniform, or kit for P.E. (exercise class, gym, whatever you want to call it). Because of the sheer quantity of things piled on the desk, she cannot use her desk for school work, and instead does all of her school work sitting on the bedroom floor. She is unable to put her schoolwork away, because the storage space we have next to the desk is also filled with her old school work, along with miscellaneous stuff that I don't know what it is but it's her stuff.
Another example: she sometimes plays video games with her friends. There is a space next to the TV that would be ideal for her games consoles, however they cannot go there as this space is filled with.. more clothes. a big ol' pile of them. Her drawers are filled up with more clothes that she doesn't wear/don't fit her - her main clothes pile that she actually to wear the clothes out of seems to be over in a different part of the room. (Until I sorted through my clothes again this month she was using MY clothes drawer as a footstool while gaming, which really annoyed me).
As much as I'd love to throw all of her stuff in binbags and say "Hey, if you want anything out of there take it out before I take it to the tip!" I think this is inappropriate (plus the konMari book says not to mess with other peoples' stuff). However it's really causing me some stress - I can't use storage because it's filled with her stuff that she's not even using. I only ever see her do 3 or 4 things in our room, play video games on her (floor-located) consoles, use her tablet (or a combination of the two), or do schoolwork (on the floor because the desk is full). So even the drawers and piles filled with miscellaneous stuff are useless. It's not a simple case of a few cups here and there - there's a whole corner of the room that I don't even know what's under it!
I've spoken to my parents about this, but they've basically just said that they can't get her to do anything either. She's used a variety of excuses to me (largely "I don't feel like it" which, how can I even do anything about that?? and also "mum doesn't want me to throw anything away" which I know full well isn't true). I have tried things like offering to work together on cleaning and sorting the belongings, and this has just not convinced her. Recently the mess has been really getting to me and it's caused some bickering between us, which I feel isn't helping my cause. So what I'm really looking for is a good interpersonal way of getting her to do it which won't result in her responding in personal insults (which has been the only way she's been communicating with me in the past week or two) or my parents getting mad at me.