AITA for telling my sister she can’t use the bathroom?

I am a twenty years old disabled person. I’m getting up at 6:30AM everyday, and have to leave home at 8:00AM to go to work. I take meds in the morning, and I have to wait 30 minutes after taking it before I can eat my breakfast (or it won’t work). I also sometimes take longer to shower since my mobility is reduced and I might need to sit (and I have a hard time washing my hair). I’m waking up extra early just for this even tho I have chronic fatigue and need to sleep longer than average. I also go to bed between 8:00PM and 8:30PM to try to meet my needs, but it’s still not enough.
So I wake up, take my meds, go to shower, when I have my underwear on I go back to my room to get ready, and only then I can eat breakfast. And I still have to make my lunch for work.
My sister’s 15 and she’s waking up at the last minute everyday to go to her highschool. She showers at night and she’s the only one with a room that can be locked. So she doesn’t need to lock herself in the bathroom or get dressed in there.

But every morning, I’m trying to go shower, and she’s inside the bathroom, locked, for 20 minutes, just to get dressed.
So every morning, I get pissed, ’cause she has other options and is blocking me from getting ready.
And when I’m trying to explain this to her (a bit annoyed I’ll admit), my sister’s screaming at me or straight up lying in my face about what she’s doing, and our mother is getting mad at ME for being mean, saying it’s not worth it and it’s not that important anyway. Like I’m dramatic.
In their opinion, it’s not a big deal, she can use the bathroom whenever she wants and I’m just impatient. But I’m trying to be respectful and let her use the bathroom for it’s intend use at the hours she needs it (I’ll wait for her to use it or go brush my teeth and hair extra early for her to be able to use the bathroom freely at her usual hours).

It’s not the first time my sister and my mother told me I’m in the wrong in this situation, so I’m starting to worry they might be right… But to me it’s just poor time management and a lack of consideration from her side.
So, am I the asshole ?

Eddit : I live in France, and in most houses/apartments the toilets are in a separate room. And the sinks and the shower/bath are in another room. That is the case in our apartment, so we can all go to the toilets freely if the bathroom is used and locked.
I do try to pack my lunch in advance, usually the Sunday for the next week, but sometimes I physically can’t do so, and my sister regularly steals my lunch when I’m not home. I didn’t mention this issued because I didn’t want to influence people, we already tried to solve this problem by organising the fridge (one shelf per person) but it still happens. I also can’t afford to eat out, I’m not financially comfortable and plans my groceries really thoroughly to be economic, meet my needs and last the whole week.
My sister showers at night, and I make sure to take my precautions before she comes home so she can get clean after coming home. I shower in the morning but keeps the door unlocked for everyone to use the rest of the facilities and do what they have to do while I’m in the shower. Everyone’s know I don’t mind, and that’s how we do it on the weekend so I know it’s not a problem for anyone either.

Edit 2 : This will probably be the last update. I had a long conversation with my mum this evening, she does agree with me but :
1) She doesn’t want to be in direct conflict with my sister or start an argument or a situation, my mum doesn’t deem it useful since I should be moving out soon
2) She doesn’t know what she could do to unlock the situation anyway since my sister isn’t listening at anything, isn’t contributing to the house in any way and has a tantrum when things doesn’t goes her way
Basically her position is *Let it slide you won’t have to pull with it much longer anyway* which I agree with.

I also want to make clear that I wasn’t looking for answers or solution since nobody in the house was open to discussion. I just wanted an outside opinion on the matter and thought here was the right place. I know I might’ve been seen as argumentative, looking for excuses or to turn the situation and opinions to my side. I just wanted to make my situation clear and that was definitely not the right way ! Our family is pretty complicated, my health too, any solution that might seem easy has either been tried or isn’t compatibility with my physical limitations (for example moving my planning around, waking up earlier, shower in the evening etc…). I know it might feel like dishonesty or lying to you but trust me when I say I wouldn’t put up with daily arguments if I couldn’t.

Thank you all for your insights, every opinion is interesting to hear and it still helped me to understand my sister’s point of view (but I can’t bring myself to fully agree with it, maybe I’m too stubborn). Thank you to those who tried to put themselves in my shoes, it does feel good to feel seen and understood. I don’t think I’ll be answering any more since I feel like I’m mainly repeating the same things over and overs again, but I’ll still try to read most of it !

Ultimately sorry if I couldn’t make myself clear or if I did any mistakes, English is my second language and I mainly learnt it on the internet, I hope it was still readable.