Any suggestions for dealing with a sibling?

I am 45 years old and with the exception of college have always been at home helping to take care of my Mother. She has been disabled since I was 4 years old when she had two large brain tumors within one year of each other. I don’t have much of a life, but I wouldn’t have it any other way. I enjoy spending time with my Mother. My Father died suddenly in 2008. He would pick up prescriptions and take her to the doctor, etc. – she has not driven since 1976. I now do all of that including, pay the household bills, grocery shopping, etc. I work 7am – 5pm Monday through Thursday. She is now 82 and is blind, deaf, and partially paralyzed on her left side. She has poor vision and hearing on the right side. She has had several mini strokes. She is mainly in a wheelchair but can walk a few feet WITH assistance. She falls A LOT, and has broken numerous bones. She needs help bathing and dressing. We prepare her meals – soft foods only, and she cannot be left alone to eat because she chokes easily. Her medications are given to her throughout the day. She requires around the clock supervision because she cannot (and will not) accept her limitations. She has early signs of dementia so some days she is confused and forgetful while other days she is sharp as a tack. She will spend hours on the toilet if you let her, a lot of time just sitting in there playing with the bathmat.
I have an older sister that moved in with us in 2009 to help out (she previously had not been at home since 1976). She works out of town for months at a time. We have a sitter with Mother when my sister is gone and I am at work – my sister pays for this and as you know, it isn’t cheap. She has been home for 5 months now, the longest period of time since she moved back home. My sister is verbally abrasive! I also believe she is about to “snap” mentally. I am so used to all my Mother’s needs and wants that I can do things without her asking for something. My sister’s answer to everything is “Well I don’t know!” She is always barking for my Mother to straighten up, throw your shoulders back, lock your knees, wipe your mouth, change your clothes, etc. It makes my Mother very self-conscious and embarrassed. She has now taken to screaming at me!
When I get home at 5:30pm she will announce that she is off duty, and she rarely helps out the three days that I am off a week. She does not have a car, so I run errands for all three of us. I get up throughout the night with Mother or stay up all night with her when she won’t go to sleep. She pays all of the utility bills, which I am grateful for, but she is constantly throwing money back in our faces. Mother pays the mortgage and numerous other bills. I pay the phone bill (house and cell), cable, internet, my car note, insurance, and gas, the grocery bill, and for Mothers out of pocket prescriptions. She keeps telling family members that she is doing all the work and that I don’t do anything. So my cousins are constantly threatening to take Mother away from me and her home – which she DOES NOT want. I don’t know what to do…how to approach her to talk. My sister can change from fairly nice to mean as a hornet in a second. Confronting her is NOT the answer, but I need to do something. Any suggestions??