Guilty or fending for myself??

So I’ll make this as short as I can..
My Mom (65F) and Step Dad (74M) are not doing well health wise. She has progressive MS and is now wheelchair bound and has been for about 3-4 years now. My Step Dad has dementia-the doctor says it’s vascular but I feel differently. When Mom and I went to his doctors appointment with him (after the doctor sent him home with a note needing us to come with) he tells us that he gave him a brain scan 3 years ago and diagnosed him then. Which Mom and I had no idea about. Of course we had been seeing him mentally decline for quite awhile it would’ve been nice to know he had a diagnosis. The doctor says vascular dementia but I say it’s LBD, due to the fact that not only is he in his own world, but has terrible paranoid delusions, incontinence, seems to only be aggressive to my Mother when I’m not around, accompanied by hallucinations of demons next to his bed at night. And btw he refuses to lock up his multiple firearms around the house. Making me scared to come home late at night. I live with them because I’m the bad twin between my brother and I. I’ve made bad decisions through my life and I’m trying my best to change. I’ve been clean for almost 10 years now but with today’s economy I’m not financially stable. I work 40+ hours a week and still only make around $2,200/mo and renting prices in my area are $1,200 and up. Then along with all the other expenses of life and not getting government assistance I can’t live on my own unfortunately. My brother lives a few hours away but is very financially stable. Good job (has the ability to work remotely if necessary), owns a 4 plex and rents out 3, along with a successful band bringing in money as well. So with me living here I’m tasked to spend almost all my free time after working 40+ hours + the commute home to caring for them. Step Dad can not drive anymore (thank God, it was BAD) and she does have the ability bc of a pricey handicap van with hand controls. Even though she complains that it’s still too much for her to deal with. She cried and threw hissy fits begging for her freedom until he allowed her to buy it and now maybe drives a couple times a week to the park down the street. They are definitely upper middle class and don’t stress on the things most people scraping by do. Own a house, 2 boats, 3 vehicles and tons of expensive stuff that is never used like tools and always top of the line brands. That now, and for quite sometime only collect dust. But he refuses to let anything go..for no reason..and the dementia has just made it worse. 85% of the time she’s in a mean hateful mood bc they don’t get along and haven’t for years and years. A loveless and sexless marriage of 25 years will do that to you.. She’s actually told me that she expects me to care for him until he dies then care for her until she dies then live off my inheritance..which is crazy..they are not millionaires or anything. And her MS is making her mental health decline as well but slightly slower than his. Yes I have a warm place to sleep at night with no rent but the constant negative day to day bs and emotional/mental stress is killing me. She wants me to quit my job and care for them without pay. And acts like I’m a POS bc I can’t handle the stress. I’m trying to get a much better paying job back in our home state that could help me have a real life of my own and not depend on them financially anymore. She’s super pissed that “I’m leaving her to be with HIM” (she says so full of hate). He is actually pretty chill just losing his words, thoughts, and in a world of his own but she is mean and picks at him all day about dumb childish things and makes it worse. I’ve begged and pleaded with her to stop whispering to me about him bc it makes him have paranoid delusions that we are against him..she won’t stop. I begged for her to use a calm demeanor and be loving, kind and understanding when he’s having a hard time but she refuses. Stop blathering to him about 4 different things at the same time, with multiple sentences about off the wall stuff bc it confuses him but she won’t stop. And always asking him “don’t you remember this and don’t you remember that” I beg her to stop phrasing things that way bc he DOESNT remember so chill out..but she won’t stop. It’s a never ending cycle of things that want me to bash my head against the wall on a daily basis. Me going to work my shitty job is my only escape but while on my commute home the closer I get to home I start to think to myself “ok what’s going to be the issue today..” I can’t do it anymore and am trying to get a better job. Not only to save my sanity but be financially stable for once in my 39 years of life but she says and makes me feel like I’m abandoning them. He also has a daughter (that calls twice a year and pretends to care but obviously doesn’t). Why can’t I live my life for once and be a real person and not have my brother or his daughter come help out for once?? A visit from my brother a couple times a year doesn’t add up to the kind of shit I deal with on the daily. He cares, he truly does but I’m sure I’ll be the one in the wrong when I actually do leave to be on my own. They make more than enough to pay for a caregiver but expect me to be here for the rest of my days or at least theirs. They don’t need round the clock care of anything just maybe 15 hours a week to check in, run errands, go to the park etc but they refuse to “let some stranger come into their house” And ABSOLUTELY refuse to consider putting him in care facility..even though here sooner than later is what needs to happen. I’m going crazy with guilt, stress, depression and the feeling of never ending overwhelmingness that is about to break me.. I’m not sure what to do..I’d feel like a bad daughter to leave them to figure it out but I’d also probably have a mental breakdown if I don’t..
Any thoughts..??