"Anyone here have any ideas or insight into how to help mom allow my sis to take more control of her care? Any ideas on how to get my mom to see that she may not be the most objective person to be making decisions for her grown daughter, given especially that she refuses to discuss cancer or my sis's case or the details of what is going on? I have reached my level of frustration and frankly if I didn't love my sister so much I'd say screw it and never talk to my mom again. Her behavior in this has utterly disappointed me."
Your little sister is your Mom's baby and there is a bond between a mother and her child that no other bond can equal. Your mom is experiencing a pain no mother would want to experience, that is: seeing a child she loves very much battling with a disease that can kill. Her not listening to you when your sis' cancer is discussed may be a way of shielding herself, but apparently she knows your sister's case well because she is the one who gets to talk to the doc when she brings her to the hospital.
I would listen to her and understand what she is and is not saying. Ask questions and allow her to express what is really in her heart and make her feel that she is loved, understood and accepted. Perhaps, if she does not sense any antagonism - she will listen to other opinions. Life is too short to quarrel with the person who loves us. Remember: "A soft answer turns away wrath" and " A merry heart does good like a medicine."