How do I care for a Mom who's overbearing and demanding?

I left home at 17- 30 yrs ago.Since my divorce I live back home (2 1/2 yrs) I have banged heads with mom my entire life. she is stage 4 terminal lung cancer. I am her caretaker and have become my fathers- since she use to take care of him for the last 17 years and yes i have helped her routinely. How do I keep a smile on my face along with my sanity ,cause i am close to losing it and it's only been 4 months. I run my own business, which is suffering. I have become a caretaker/homemaker. (Was married 25 years and my wife did it all for me-yes I gave up a good thing) Cook, clean, iron. I do it all now, with no help. My problem, I don't wanna stress my mother out. But she is demanding and expects me to do it all. example if I am shower my father I have to stop to tend to her needs, be a simple glass of water,a pill or hand her the phone. i love her very much. She is very confrontational and I am the only one in the family that will speak up. She's controlling and loves to give orders. Doesn't ask but tells you to do something. My sister lives a 1000 miles away and my brother, well lets just say his last statement was" thanks to you screwing your life up now I will have to take dad" so you can read in to that statement. To me he want's no part of helping me out. My mother is 68 and my father is 70, he is in good health aside the fact he suffers from anoxia (lack of oxygen to the brain) from a cardiac arrest in 1997. He has short term memory problems, walking and balance and he confabulates. ( makes up stories that sound perfectly plausible) but he need 24/7 care. So I guess the real question is how does one do it all by themselves, try to work and of course keep a smile on my face. I don't mind doing it. It's just nice to be asked and not told or it be expected out of you. Yes I have discussed this with my mother, she doesn't see it as a problem. Her solution is always the same " I will pay someone to cook for me --I will pay someone to clean for me--and she has and they just rip her off. Really ready to lose it in Pa. Thanks