Who's in the wrong here?

Her family isn’t going to help her. I doubt she expects they will. She was just short with her niece because she was exhausted and irritated. She wants her niece to apologize and this to blow over.

Sure but so what? The world is unjust. OP has just found out that what you get when you work your fingers to the bone is boney fingers. If she wants more help from her (seemingly hideous) family she is going to have to ask for it—demand it. And she might find that they don’t step up. But at least she will know.

She’s in the total wrong here I think, and thank whatever you believe in she’s gone.

if you’re a guest you should respect the home habits. 11.30 is not 1.30am. And so on. She’s the passive aggressive here, tears instead of apologies for lack of consideration for your ospitality.

Entitled immature behaviour all the way

What more needed to be said at that hour? Do you think an adult really needs it spelled out that coming in 2.5 hours late to a household where someone needs round the clock care is selfish, rude and inconsiderate?

No, she knows and is ashamed and won’t face it.

So she doesn't come to stay to spend time with you? It's because you're convenient for her to see her mates?

”I am exhausted “ is extremely indirect. If you or the OP think it was a cue to the niece leaping into action and taking better care of OP you are very mistaken. That is why some of us are calling OP passive agressive. She was not direct and forthright about what she expected from her niece and her statements about her situation were ambiguous. Niece thought things were fine and that OP was waving hello when OP thought she was drowning and waving wildly for help.

dear OP, I am so sorry for the screeds and screeds of unsympathetic feedback you've had on here. I hope that the critically ill person you've been caring for is now in a better position such that you, yourself, can catch up on sleep, rest and time to yourself. I also hope that your neice will take time to reflect on her (selfish) behaviour and apologise to you for her thoughtlessness
Maybe it's time to start putting some self-protecting boundaries around her availing herself of your hospitality? By all means, if you want to, loan her the "guest key" to let herself in.

She's 20. She's an adult. She's lucky to have an aunt she can fetch up on at a whim.

If @Whyohwhy1974 hadnt slept for 48 hours and then had a mere 4 hours sleep… I hope that the trip would have been cancelled anyway as driving would be dangerous.

although odd to have even arranged a day trip when in the immediately preceding days the op had got zero sleep due to dealing with the “life or death” crisis of a relative

She was out with her mates. Who would have "feminine supplies" in their homes too. And pub loos still have tampax machines.

There's nothing that couldn't be managed or borrowed until the next morning.

Might be less comfortable than she likes, might not get to chsnge her knickers til next morning, but them's the breaks if you change the stakes.

It's ridiculous to mollycoddle her on everything and then wonder why she didn't put OP first.

She was alluding to the niece needing feminine supplies. Not a toothbrush. I agree being more direct would help but can’t blame her for not being ready to deal with this unexpected change in plans.

Neice also didn’t call and say “Would you mind if I come back later? I know it means you have to stay up longer.”

I don’t think it’s easy in the moment when someone presents their plan as a fait accompli.

OP wasn’t passive aggressive, she was quite direct. “I’m exhausted.” Did she need to say “Hey, this was uncool, you know I’m at a breaking point. What the hell were you thinking coming in so late?”

HAHA! No “hierarchy, no respect!”

What good does that do OP? She wanted to, snd had, a loving and close relationship with her niece and she has blown it to smithereens by her actions. Her niece left and she doesn’t know how to repair the relationship. She came here to get lots of pats on the back and reassurance that she is right and niece is wrong but that is some cold comfort when she is now estranged from someone she loves.

Would she rather be right? Or happy? That’s the question.

No hierarchy, no respect. Of course she is in the wrong. Your house, you are older, she has been snarky to you in your own house and you are asking who is in the wrong?

So, have you spoken to her to try and fix things OP?

Given your responses to me, you seemed to want to.

She was wrong in the fact she went along with the niece yet resented it, and then was passive aggressive about it.

When niece said she was going to be later than planned, OP should have just said "no, please come back I need sleep" or "no, you'll have to stay with your mate instead".

OP's reason for not doing that is her DN had stuff at her place 🙄 (but she's a young adult who could perfectly well sleep on a mates sofa for a night without a toothbrush and come back in the morning).

I think OP was people-pleasing to her own detriment, but seething about it, which is really unhealthy. If you act like nothing's too much trouble and then boil over, it comes as a nasty shock. Far better to have some boundaries in the first place.

Alternative perspective, niece knows she messed up, was shocked Auntie had reached the end of her resilience and wasn't all sweetness and light come the morning, ran away because she didn't know how to handle it and is feeling massively guilty for adding to the problem, and now doesn't have the emotional maturity to offer an apology to get things back on track.
Total reach on my part, of course.

I’m really curious at how the OP was wrong at all? She was a little short when an adult made her stay awake for 2.5 hours behind the time they committed to be back.

Who would be happy with this type of inconsideration?

Well, I’d be livid and looking for an apology from her. Is she normally more considerate? Because changing the time then being even later was extremely inconsiderate and selfish of her. Obviously you’d be cross.

But going forward, learn these words: “That doesn’t work for me.”

So it’s:
Her: ”I’m going to be later.”
You: “That doesn’t work for me. I desperately need sleep and it’s not safe to leave the door unlocked. You can come get a key and go back out, though.”

I think I’d leave this for now. You’ve done so much for her and the relationship should be strong enough to recover. If not, well if she falls out with you because you were short when she had been terribly rude, then she lacks character and something was eventually bound to happen.

If someone is staying the house you should probably give them a key.

You’re both wrong but you more so. Sounds like it wasn’t the best time for you to have guests. Hug it out! You can move past this and communicate better in the future.

I can see OPs frustration here, a lot of questions are being asked that she’s already answered more than once.

I am guessing this isn’t the first time you have had tension over something. You are both in the wrong. Her for not taking a key and you for not telling her to take a key. Why on gods earth do you need to commit to stay up to let her in? Especially if you are exhausted as you say.

I d reach out.
It not so much her pretty normal behaviour for a young person, it's more you"re sleep deprived and stressed. But don't let that get in the way😁...reach out and mend things.
In essence she sound like a lovely person.

I think this was on you. Why would you wait up for someone who is that age? Why doesn’t she have a key?