How can I get my mom to visit my sister's for a weekend, to give us a break, she never wants to go?

Great idea to get respite time. You not only deserve it, but, it's necessary to maintain your strength. I would consider a couple of things though, so you can make it work. I'd question if the reason your mom isn't interested because she's afraid. My LO was very apprehensive about going places. Even places that she was familiar with. Their memory may fail them, they get confused, they get scared. Anything outside of the normal may scare her. I'd be sensitive to that. Can sister come to her place? Can they set up a room at sister's that looks a lot like her regular room? I'd try to accommodate her as much as possible during this stage. Later, she may not mind or realize she's somewhere different.

Well, she goes, whether she wants to or not. You need respite! And good for your sister! Don't make it a question. "Mom, Sis will be here at 8:00 am on Saturday and you will be spending the weekend with her. Hubby and I need some time alone." Argument? "Hubby and I enjoy having you here, but we also need some time alone once in a while. I'm sure you will enjoy a stay with Sis once you get there." Don't give her a lot of time to fret about it.