I went through a very similar situation before my mother passed away. All of my four siblings are in other states and I lived an hour from our parents. The last three months she was in a nursing home and I went once or twice a week to visit. My siblings often called my father to talk to him and get updates. Yet rarely reached out to me to see how she was doing or how I was handling it until the very end.
I think if you just make it a point to reach out to your sister and see how she is doing, not just get updates on mom she will appreciate it. Obviously you care about your sister's well being and supporting her. Maybe send your sister a handmade gift, card or copies of old photos just to let her know you are thinking about her too.
Best of luck with your mom!
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