I really feel for you. It's utterly awful to be expected to help out/care for parents who abused, used, humiliated, shamed and controlled you. The resentment of having to give more of your life over makes you so angry you could burst. The entitlement of these parents who when they were young and strong treated their vunerable children like utterly shit but when they get old and vunerable there is outrage if they don't get the best treatment, everyone's attention, treated like someone really special.
My dad died a few years ago (not abusive himself, but very much an enabler/supporter of my mums abuse). Since then I have helped out with my mum (been paid to do it) and quite frankly it probably wasn't a wise choice (I had sort of forgotten how bad she was as I've been away from home for over 30 years and thus not been her punching bag for a long time). Despite being paid the anger I have felt at the treatment I've been put through. Yes my mum had a bad childhood herself but has got to her 80's and still thinks she was a perfect mother and can't understand how her children are all so terrible, useless, want nothing to do with her. No self awareness whatsoever. I have aged horribly since I took on this role and do regret it.
Now social are involved to put her in care home as she needs 24 hour care and I will have to hassle of clearing her house which is very cluttered, selling it and cancelling all the services etc. After that I plan to do pretty much nothing (obviously I will no longer be getting paid). If the carehome need people to take her to appointments etc I won't be doing it. They can charge her extra and take her.
I feel like it will probably take me about 5 years to recover from the experience.
I totally understand your conflicting emotions of resentment, guilt, rage, feeling sorry for your sister.
I think you have to accept your sister chose to get involved and that was her decision. You don't need to. Hopefully if your mum has just gone in carehome the work will tail off for your sister and she can get her own life back.
I sometimes felt angry that my sister would not help (she got offered the chance to help and get paid too). However I understand why she did not. I needed the money, she did not so sensibly did not get involved. I think I would have felt guilty to let my sister deal with it on her own but I have a personality where I have been brainwashed to put everyone elses needs first. This is wrong of course and you should put yourself first.
My mother of course played me off against my siblings when we were children (called triangulation and very common with abusive parents) so I have no relationship with my siblings bar very superficial. They are essentially strangers to me.
These horrible, vile, abusive parents who keep on living on and on and on and bloody refuse to die. Costing the NHS absolutely thousands, complaining to everyone, calling ambulances at drop of hat, draining the life from their children once again.
Mine tells me often 'I wasn't worth the pain of childbirth' amongst other delightful sayings that she trots out regularly 'why have a dog and bark yourself'.
She fully admits she had children so they would look after her, pay attention to her, make her feel better. As a child I got good grades, never had unsuitable boyfriends, didn't smoke, drink or do drugs and had to tell my mum everything. Information was power to use for her.
Don't feel bad for how you feel. I pray daily for mine to die. She is terribly worried about dying. Frets constantly about it. I reckon she thinks her day of reckoning is upon her (and going to hell etc).
Hugs and sympathy.