My Dad has had this problem with his (much younger) half brother. There's a 25 year age gap and he's only a couple of years older than me.
His brother is almost certainly autistic and when his mother died he was left living in the house which he can't afford. His mum left him £8000 which he thought was an enormous amount and he'd be able to survive for years off it (which gives you an idea of how dependent he was on his mother).
He struggles to live independently. He's in his early 50s. The trouble is without a diagnosis he's rather screwed.
My parents eventually persuaded him to put the house on the market but he's now been on for twelve months because he won't drop the price because the reality is he doesn't want to move. He will eventually lose the house if he carries on. It's in a state though because he can't look after himself.
He lives more than 200miles from my parents who are in their 70s so not in a position to really help. They are supporting at arms length because they realise that a) the age difference means he may become my problem if they help more b) he almost has to fail before social services can step in and help him with supported living because he will end up with an assessment.
The whole thing is ridiculous in a way but they are powerless to do much more.
My dad no longer is taking his calls for everything under the sun (including how to deal with the smoke alarm when the battery is flat and changing a lightbulb). My mum is much more patient so she mostly deals with him.
My Dad had conversations with his Dad and Step Mother for years before they died. They were largely in denial and didn't do anything. He has found it enormously frustrating.
I do think you have to make a conscious decision about what's in the best interests of everyone here - including yourself.
The reality is you have a husband and kids and this means there's limits to how much you can take your sister on and given she has a history of drug abuse you have little choice but to keep her at arms length.
Even if she stays in the house, if she has no income she will not be able to stay in it because she won't be able to afford to maintain it, run it and pay council tax with no job. This is your conversation starter with your parents about what provision she might need. The reality is she will be better off if the house is left to you both precisely because you can force a sale and this will force her to down size to something she can maintain and afford long term.
You can not get involved too much because of the implications to your children. You will need to be prepared to go legal if she harasses you for this reason too.
In terms of what support she might get, if she has assets, she will get little support before she's lost it all. That's one of the frustrations my Dad has that he has limited options available to his brother until that point because he is refusing a diagnosis pathway. My Dad has put a few things in place but it's not really enough. So again, leaving the whole house to her probably really isn't in her best interests because that just delays access to support she may need as there will be no identified problem if she's off the drugs.
So even though you might feel obliged to try and help your sister because really careful here about how it will impact your family and whether that ultimately will help your sister. You have this situation where letting her fail without your support may be in her best interests long term because you don't know how long either of you will be around for. The last thing you want is for her to become a burden to your survivors if you go first.
I wish I had some solutions. My parents really don't have any apart from waiting for the inevitable crash before they can help his action / access anything else. They understand this and find it hard but don't know of an alternative that will work for the rest of his life. Their age makes this particularly concerning though my uncles lifestyle is so poor and his health not great my Dad may yet outlive him (my grandfather lived to 95).
I don't have any contact with him - my parents have made a point of telling me not to now with all of the above in mind.