Who's in the wrong here?

I don't think you were in the wrong OP. She said she'd be back at 11 and was the and a half hours later. I dont think you were passive aggressive and I don't think you were wrong. She's 20, not a child, and should understand you would be annoyed if you got even less sleep because of her.

also, if she comes back, give her a key

This could all have been avoided if you’d given her the key.

Perhaps don’t have people to stay if you’re on the verge of a breakdown.

It’s your responsibility not to take on more care than you can cope with. If it’s too much then it’s time for agency care or a care home.

There are some people like the OP that will not help themselves and will not do the most logical thing ie don't invite people around when you are exhausted!

And when you tell her not to invite people around she says she didn't invite them around, they asked, which is exactly the same thing but they can't see it!

I have known people like this in real life and I just stop offering them advice because they never listen and they do what they want and they just moan about it!

Of course she is in the wrong! She is 20, an adult who can do what she likes, but she agreed to a particular time, then messed you about, and then turns on the waterworks like a toddler. You were doing her a favour, and she's selfish.

Yes, in hindsight you should’ve given her a key but I’m agog at the responses making out the niece to be some precious innocent child who can’t possibly be expected to show a bit of courtesy to her exhausted aunt and will now be spiralling that auntie doesn’t love her because she snappily said she was exhausted. If that’s how the niece is feeling she needs to get a fucking grip! Plenty of people her age live in shared accommodation and have to deal with inter-housemate relationships and difficulties. If they’re flouncing off in tears every time someone’s pissed off over a forgotten key or other annoyance I despair for that generation.

Only skimmed OP's posts (which include quoting).

OP, last Saturday DS 17 was meant to be staying at a mate's.
At 3.30am I was woken by the light of my phone.
He'd decided to come home, no key, house locked obv.
I let him in, went straight back to bed muttering and very cross.
He apologised profusely the following day (when I was still cross).

His 17 yo brain isn't so unformed it can't work out it did wrong and apologise.

Of course, he doesn't really have anywhere else to go, so no flouncing off anywhere.

Not to defend her because reaching those depth of tiredness is awful, I don’t think your niece had any idea of how hard things were/are for you.
Actually I don’t many posters on this thread do either unless they’ve experienced it themselves.

Your niece will have heard exhausted and thought ‘oh yes like when I’m getting up,after a night out’ or ‘when xyz happened and I didn’t sleep very well for 2 nights’. Not the exhaustion you’re describing.
It’s very hard to convey what it actually means. I’ve been tried like this and even when my dcs were babies and not sleeping, I never reached that level of exhaustion.

On that subject, I would assume people haven’t quite grasped how bad it was/is for you.

There are many many posters like me who say that the OP is not in wrong.
Why should she listen to you rather than the majority of people (as per the vote)?

Also I love the position that only the OP is an adukt and that a 20yo should be managed and things smoothed over for her. I have a 20 and 22yo. Both working. I can tell you no one at work will take a soft approach and no one would have a problem with a curt ‘im exhausted’ answer. They would be getting in with it and, as a mother, I’d exiect them to think about why they got a curt answer tbh.
Isn’t that part of normal life?

Given the state you describe yourself as being in… sounds as those this relative needs much more extensive help than an at home carer is appropriate for

What was the day trip planned?

You’ve asked which one of you was in the wrong, but you’re not listening to anyone who tells you it was you. So you obviously think you’re right and that’s that.

Your exhaustion and lack of ability to say no is not your nieces responsibility or fault. If you weren’t so exhausted you wouldn’t have reacted in the way you did, to what is a very minor thing. It’s incredibly hurtful when your loved one takes their issues out on you and thinks it’s absolutely fine because they’re struggling so you should just have to suck it up.

You’re the experienced adult in this situation, you should have been able to have a calm conversation in the morning.

Your sister is also in the wrong for not helping with your relative. Why isn’t she helping? Is this a permanent arrangement, or will relative be going home?

You keep saying you are in a dark place and yet when people suggest talking to her and putting boundaries in place you have loads of excuses like "well I didnt invite her" etc This is your home. You arent without agency here.

You cant have it both ways. If you are struggling then bloody tell her she cant stay. Good grief.

I’m guessing that she has gone out with friends while staying with you in the past. Was she ever later than she thought she’d be? I suspect that she thought half an hour isn’t a big deal and that your level is tiredness is akin to her staying up until 2 but making it into school or work the next day sort of tiredness because she’s not experienced what you have.

You mention that you are “patient with her” Does she tend towards unreasonable/immature/annoying behaviour normally? Does this manifest as you overlooking stuff she normally says and does because her home life or other life circumstances aren’t great? You aren’t unreasonable not to have any patience when so sleep deprived but if your relationship norm involves you overlooking stuff then she’s not unreasonable to be thrown by a different dynamic from you. I’m not saying that you were unreasonable but she’s not totally wrong to be shocked that you weren’t your “usual self” We also weren’t there so can’t comment on your tone or body language.

Plus, it's not really a host situation. She stays whenever she wants.

Then why hasn't she got a key? it sounds like she just stays whenever she feels like it so why did you not give her a key and instead, insisted on waiting up for her? so weird

I feel like this issue is being exacerbated because you are treating her like a child when she is a fully grown adult.

You staying up to 12.30 was out of her control as was you having to wake at 6. She’s inconvenienced you by 1 hour - hardly worth falling out over or declaring to her that you’re exhausted as if it’s her fault.

I think there’s more to the flouncing than just this incident.

Hope your life gets easier and you make up very soon and put it behind you.

No, you can't travel back in time.

But you can stop letting it create a wedge between you and someone you love who has been inconsiderate but is young and loves you too.

If your home is her home she should have a key, she should be able to come and go as she needs when she is staying. Also you said you were up until 12:30am with care responsibilities, so would've been up anyway. It's also very strange to have someone stay so regularly who you don't give a key to

She probably said it in all good intentions.

And she let OP know she was going to be later.

The waiting up for a 20 yr old on the lash was ill-conceived from the get-go.

Some really good points here so I'll add that at her age she probably doesn't know how to cope with you feeling like this. You probably also don't realise how hard you've been to live with.

She's 20, of course she wasn't going to be back at 11, though.

Then she shouldn't have said she would be.

I agree.

Maybe you do need space right now.

Don't worry about this, it will blow over.