Don't let your sibs or anyone (especially your mom) push you into doing what you don't "want" to do or guilt you into anything, especially because you're not married/don't have a family to care for or whatnot. You have to live your own life, can't live your mother's or sibs and you have to give yourself space to do that.

That being said, you obviously know what is "right" (that your mom get professional/assisted care) so stick with that. Get on her shite list if you have to by backing up whoever was brave enough/energetic enough to set up the Medicare thing. If you all "gang up" on her together, it will be much harder for her to do her own thing. If you can find one of your sibs you "agree" most with, kind of hang with them instead of so alone?

My stepmother needed her car taken away but wouldn't listen to a woman so my stepsister and I had to bring in one of my brothers to do the dastardly deed. He flew in from Hawaii to Maryland for a week and we got the car away. But boy, when he went back home. . . different story; my stepsister and I got constant calls from mom telling us how mean, evil, ugly, etc. we were. It was much better for me, knowing my stepsister was getting the same treatment. Can you call/"unite" with your sibs and everybody have a single "party" line and "assign" a couple of you to talk to doctor's, etc.? We were lucky because my mother went to military medical facilities and my stepsister had studied to be an RN, and went to work part-time for an assisted living facility, etc. and things worked out. But it was never "fun"/enjoyable. I recommend treating it like a chore instead of taking it personally; I was really lucky because I was seeing my T the whole time because of my problems with my stepmom so that helped a whole lot working through all the shite :-)