Dependent sibling and elderly parents. What what happens when parents are gone?

Could you start a conversation with all of them based on an imaginary friend of yours in similar circumstances where it has gone sideways and say 'what was everyone thinking is going to happen as this has taught me its good to discuss now' or is it the kind of situation where your sister is always too... stressed, and wont want to discuss at all?

Agreed. Start hardening your heart now, you are NOT responsible for your sister.

I think your parents wont have it in them to drop the rope on her, she wont suddenly be able to be a responsible adult when they pass. Id be prepared to have zero contact for your own health when your parents do pass.

Yeah if it's an owned property I'd gently ask questions about the will. My friend had a very difficult situation where his parents left the family home to the two of them, brother had never moved out and thought that because it was 'his home' he should be allowed to stay in it without any recompense to his brother.
It was a London property so worth a significant sum, brother didn't work and his stance was 'i can't buy you out without money I don't have so tough'. My friend had to take legal action to force the sale of the house, which was stressful as well as expensive.

I’m in this position. It sucks. There is no help and the help there is they have to want to access.

My guess is she wants the house when they die

If she's eligible, get her on her local housing register now. She won't be considered in need now, so won't be offered anything, but if she were to become homeless when her parents pass, it may help her then. Whilst the housing register isn't a waiting list as such, time on the list does count within the banding, so if she were later given some priority for bidding due to homelessness, time on the list would count compared to others in the same situation.

If your parents are in a council or housing association tenancy I'd check the succession rules to make sure she's not banking on inheriting a tenancy she isn't eligible for.

You might also want to find out whether there are any supported accommodation services in your area or nearby that she might qualify for (there can be some for those considered 'in recovery' though less than there used to be) Just if it helps you feel more confident about a stance of signposting and not allowing her to move in. Definitely don't let her move in unless you are happy for her to stay permanently, as you'll likely end up in the same situation as your parents.

I think you would be doing right by yourself and sister AND parents if you can make it absolutely clear to them now that you are not willing to have your sister live with you when one or both parents have passed.

Remove any hope that you would change your mind when it came to it.

That way they should start planning now for the future.

No need to feel any guilt - she sounds very very difficult

I sympathise but can't offer much advice. Had a similar situation with a sibling who ended up living with mum after dad died. Sibling had addiction issues and was rarely reliably in work. I worried a lot about what would happen once mum died but sadly sibling died first (very unexpectedly).

I'm not sure there is much you can do other than encourage your sister to move out while your parents are still around to offer support but if they won't take that suggestion on board sadly there's little you can do.

I think you will need to provide more info about your sister’s situation, regarding her reliance on your parents.
She’s lived with them ‘for many years’: so, prior to this she lived independently? Is her living there based on an a past economic necessity, or is there any degree of actual incapacity there?
Any addict, sober or not, of course has a permanent illness, but that needn’t be read as a mental limitation.
I imagine she anticipates remaining in this house after your parents pass away, though that would not be a simple transition.

One parent will, God willing, remain, and companionship will be welcome. Or maybe the house would need to be sold, to fund a care home place for them.
That’s a scenario whose possibility you could broach with DS; what would she do if she lost her home in this way? Should you both be looking for a flat for her to rent, sooner rather than later, etc?
(Maybe the house will have to be sold anyway, if it’s willed to you both equally.)

It might ease your parents’ worries if they knew you were being supportive with her finding her own place.