ANGRY! ( and I've fucked up)

Definitely. I'd email so there is an audit trail. Keep records. Check any bank accounts and make an inventory of valuables. Make sure you communicate in writing and raise concerns this way too.
I have friends who have been completely screwed over by greedy siblings basically stealing assets and hiding money which meant the parents didn't get the care they deserved initially

Your sister got in there first. You now know the type of person she is. If your parents don't have capacity, then, depending on the law in OZ you could seek legal advice, as she's potentially stolen the vehicle.
However if your parents have capacity, then there is nothing you can do. It's ultimately their choice.
I think you should be prepared for a lot more grabby behaviour from your sister. I assume you are both executors to the will, should there be anything left over from care home fees? (I know that must sound mercenary, and I hope you parents have a long, comfortable and happy time in their care home) but I think you need to be prepared for unpleasant surprises once your parents have gone.
Tread carefully op, if you upset your parents, you'll be seen as the bad guy, but try not to let your sister operate a lone, when it comes to your parents finances, and make sure you have a copy of all their finances, to ensure your sister doesn't skim anything else off. I think you can assume from here on in you and your sister won't have a good relationship, and once you're no longer tied by your parents you'll be no contact.

Did anyone in your family KNOW that your son wanted to buy the car? Aside from you, I mean. Did your parents or sister's family know that an offer was in the works?

It's a very different thing to take the car knowing a grandchild is trying to scrape together the money to make a fair offer vs just borrowing a car that is otherwise sitting idle.

Wormfanclub
The car was taken under false pretences from vulnerable people for whom she has power of attorney.
The parents have become a resource to plunder, which is far more important than who had what as a child, and who gets 'presents' (it was a loan) and who does not.

No I don't think it's OK that he took the car. But then, would your sister think it was OK if your son had it? The fact you don't get on with your sister is clouding this for you. Your parents being elderly and frail is the important thing -be nice to them, one day they won't be here and you'll miss them. Let the issue of the car go, you're the only one it is stressing

You may not believe it, but I for one genuinely wouldn't mind, as wouldn't a few others on the thread, by the sound of it. Surely better that than sitting on the drive gathering dust.

If the car had been given to your son, it wouldn't have been sat on the drive "minding the house" either, so that reeks of double standards, really.

It's not really about the car. Op's posts are simmering with rage over past issues and her sister's conduct and it's affect on her earlier years ..... I think.

"They are saying things like we always had dogs ( when I was never allowed one). They are denying the fact that we spent our life tip-toeing around my sister "be quiet , she's asleep, come and help with the grdening"."

These are the parents you describe as "very sweet". I don't think this is about the car, or really about your sister, but the people who prioritised her over you and continue to do so.

You can and will feel angry, but IMO you would be better advised to step away a bit and protect yourself.

This is so annoying - message her and ask when the car is coming back?!

How much is the car worth?

Maybe that’s why you’re getting conflicting comments here.

You’ve described it as being used as a run-around / a car a teenager could afford. So I’m picturing it as only worth a few grand at most. I really don’t see why that matters. It’s a small drop in care home fees.

Your sister is clearly not very nice. But you know that already. It’s not worth getting worked up about an old banger. It’s certainly not worth phoning your parents and having a go at them, saying oh your grandson doesn’t have a car, guilt tripping them. At their age. It doesn’t matter if they don’t know which city your sister partner drove it to. I think you need to take a big step back from the whole thing.

marmoet · Today 08:38
It's joint. ( I only know that as my mum gave me some paperwork and I spoke to the lawyer). Theoretically, she can't do a thing without me, but that hasn't stopped her yet.

And it won't. She may not empty the bank account, but 'birthday presents' 'gifts' for her children.
There's lots of sneaky, but just about legal ways that they can denude your parents of their goods.
I take back wait for the next thing - take legal advice on this, and get her removed from the power of attorney.

I've only read the beginning of this thread so hopefully kinder posts prevail.
You have all my sympathy @marmoet .
Parents , siblings can be v stressful and very visceral, basic emotions arise. Throw confusion and moving into a home = nightmare.
It's not so much about the car, it's your sisters actions and what it represents.

But if we asked and your parents said yes, we have permission. You might not like them giving it to strangers but it is still not theft! He is their SIL. They said he could take it.

I think the bigger issue here is you and your sister hate each other’s guts and it’s therefore making a stressful situation even more stressful. Not sure what to suggest, other than the nuclear option of you giving up your POA and walking away to alleviate the angst but I can’t imagine you want to do that.

Which of you is managing parents bank accounts. Because if it's sister you need to get onto the bank asap to ensure you have full visibility of everything that they are spending.
Don't have adult social services or similar to ask for advice how to go about this? They will have seen it all before. But hurry before the house is sold to them for a pittance.

Given that your sister has previous 'form' for being overly entitled, please ensure that the estate agent and solicitors are aware that you have joint POA and need to be consulted on any actions and financials.

I get you OP.
As per usual some MN don’t get it and are just jumping on to be rude and make themselves feel better about their lives perhaps!
Anyway your sister sounds like a CF. I’m not surprised you’re angry. Your parents have enabled her all her life and she has sat on her pedestal quite happily with her ridiculous expectations. I have a similar issue with my older sibling (who I’m NC with as much as possible) and it really grates!!
I guess you’re going to have to give up on the car situation for your own sanity. I hope you’re able to resolve and deal with future issues but it’s not easy.

Absolutely this

One thing to remember with POA is you are beholden to act in the best interests of the DP
Not what's best for you.
That is abuse.
Can each POA act alone or is it joint Op?

But I agree with you op they took the car under false pretences and don't plan to give it back anytime soon.

This is precisely it. They did not want to put the miles on their car.
OK you might argue that the car was just sitting there, but they have taken advantage of their parents infirmity.
If they were still in their home, you would not borrow the car, put a massive amount of miles on it without a very good explanation, and not return it.
What will they ' borrow' next from the parents?
What signature will they ' need' next?
When you have power of attorney everything needs to be squeaky clean.
OP I'd let this one go, then if anything else dodgy happens apply to have their power of attorney removed.

This could be an indication that there’s a lot more going on behind your back than you realise. For instance, are you sure you both still hold joint POA? Is there a new will?

In my experience there is little you can do when a scheming sister manipulates elderly vulnerable parents.

The law might be on your side, but getting justice involves spending thousands on solicitors with no guarantee of success.

Walk away from it OP, live your life in peace. She has to live with what she has done, you don’t.

So why didn't your son?

Family dynamics are tough and we all have baggage. This situation isn't going to get any better and is it really worth fighting over a car?

Maybe he didn't want to drive his expensive car hundreds of miles?

You need to take a step back. He asked, they gave permission.

Not sure why you’re getting a hard time here, OP. Seems to me that your sister and BiL are taking advantage, if not seriously taking the piss. If they had needed to borrow the car on a temp basis, because one of theirs was e.g. being repaired, that’d be different.

If your folks no longer need the car, I’d have thought it should be sold, to add to the nursing home fees pot - assuming they are paying, or will have to pay for care at some point.