I want a relationship with my parents that doesn’t involve my sister - AIBU?

[Endofanerainourtime]

I’m early 50s and have reached a point where I honestly don't know whether I'm being unreasonable, or whether I've just finally had enough.

I have an older sister also in her 50s who has had serious problems for many years, including a long history of drug addiction. At one point things were extremely bad and there were frightening incidents involving drugs, drug dealers, my parents' house being damaged, aggression towards my mum, rehab repeatedly failing, financial problems etc.
My parents have always been very involved in trying to help her. They are now in their early 80s and my sister lives with them and relies on them heavily financially/emotionally. They have always been very protective of her and tend to rescue her when things go wrong.

I have spent years being dragged into this. When my parents went away I was often expected to keep an eye on my sister. I've picked her up, searched for her, worried about her, lent her money when there were drug-related problems, had her at my house etc. My Mum also constantly talks about her when I speak to her and she rings them when we do have time together without her. There have been so many incidents over the years that I feel emotionally exhausted by it all. Recently something in me just seemed to switch off. When I hear about another problem with my sister, rather than feeling compassion, I feel numb and angry and think, here we go again.

She is also completely unable to take any criticism or challenge without going off the deep end. This has caused problems in multiple jobs and she has been through a number of them as a result. She can also be extremely nasty towards my mum when she is angry, but then will often retell what happened to my dad in a way that makes her the victim, and my dad tends to believe her version of events.

This is one of the things I find particularly difficult. I feel as though my mum gets the brunt of her behaviour, while my dad is then brought into it and ends up thinking my mum - or sometimes me - is the problem. I have spent years watching this dynamic play out and trying to stay out of it, but increasingly I feel completely exhausted by it.
The latest situation is that I told my parents that | want a relationship with them that doesn't revolve around my sister. I don't want to discuss her problems anymore. I don't want to be involved in monitoring her, rescuing her or being expected to accommodate everything around her.

also said that if she comes to my house and behaves badly or starts an argument, I will ask her to leave. I am not saying she can never come to my house. I am saying that I won't tolerate being treated badly in my own home.

My parents reacted very badly. They said I was being unreasonable, asked why I had "started all this now?” and repeatedly suggested I have counselling. My mum has become noticeably colder with me. My dad is also very angry and I've barely spoken to him.

The strange thing is that I don't actually want to fall out with my parents. They are old, I love them, and I don't want to waste whatever time we have left.
But I don't think I can go back to how things were before, where I felt I had to tread on eggshells around my sister and constantly manage everyone's feelings.

But I am struggling with the fact that my parents are colder with me and seem to think I've caused a family breakdown.

I'm particularly interested in whether people think I'm being unfair to my parents, given their age, and whether I should be making more effort to repair things or whether I should just allow everyone to be unhappy with me

I don't think they will change now, it's too entrenched. Everything and everyone must pander to the needy sister. I know how this goes!
You wanting to set your own boundaries with them regarding her, or even just to tell them how you feel, will be seen as selfish. They are martyrs really, don't get dragged in. It's very hurtful to be seen as the one who needs no help. You're just as important as she is. They won't like being told that though, as you've found.

They are probably also worrying about what will happen to your sister when they are no longer around and probably just assumed you would step in, so your recent attempt to distance yourself from your sister has probably come as a complete and unwelcome surprise to them.

What @Createausername1970 said.
This was my first thought.
They probably assume you will be defacto carer/support worker for your sister when they are gone.
I would make this very clear now.

As your mum has let you know in advance that she'll be completely ignoring everything you said and will bring your sister to your house, I think you should be out at the time you invited your parents. Let them arrive and go home again.
You cannot dictate who they have in their home (not suggesting that's what you've said to them), but that works both ways.

I would have made it clear your sister isn’t invited and if they don’t trust her alone in their house then they don’t come. They don’t get to decide who comes to your house

I love my brothers and have a good relationship with them (we're all in our 40s) but they're both currently out of work and I regularly hear about their trials and tribulations when we catch up.

I don't get invested and treat their lives more like a soap opera. Is there a middle ground where you can hear about some, change the subject, offer little in the way of opinion or encouragement to talk more about her?

Unfortunately I think that at your parents age and after what I assume is many years of this dynamic with your sister they are all very unlikely to change. I am all to familiar with the 'why are you starting all this now?' line.
I think you need to act now to protect your future self. What would happen if/when one or both of your parents are alive but no longer able to support your sister as she is used to? Dealing with emotional blackmail/pleas ' from an unwell and vulnerable parent to 'just help out your sister'/'I'm so upset about your sister' etc is really hard. Far better to get the issue out in the open whilst your parents are relatively well.
I'd also think about your parents' future care and finances, and to what extent you are willing to get involved in that. Not necessarily to discuss with them, but so that you have headspace to consider it before any crisis happens. My Mum currently really should be considering residential care due to her needs, but she and my sibling won't even think about it- largely I'm sure because that would mean selling Mum's house which sibling is relying on becoming his. This means a lot of pressure to get involved in care/medical appointments/help around the house etc. I've discovered that there is very limited money available to pay carers etc and not enough income to cover running costs on the (large) house since several of our parents' pension funds have been cashed in and this (plus savings etc) used to fund support/bailing out of sibling over the years.
I am also, as 'the sensible one' expected to help manage finances etc and am grappling with how to get a power of attorney in place (mum is in the early stages of dementia) when Mum wants it to be joint with sibling (I won't do this) and sibling thinks he can have it and just use Mum's money etc as his own. I wish I had foreseen this as a possibility and had the discussion when DF was alive and Mum was able to properly think things through. At least then if they wouldn't discuss it properly I would have felt I could walk away from it and they knew what they were doing. It's all a huge mess that I know was created over many years of our parents feeling they needed to do whatever was necessary to keep sibling out of trouble and prevent any upset/stress in case this pushed them back to addiction.