This is a tough spot to be in. Caregiving is an exhausting and often frustrating job. Your sister sounds like she’s under a lot of stress and needs some resources. You might be able to offer her an afternoon off, or “gift her” with some reading material (see: Book Review: The 36-Hour Day – SJL DRC (stjohnsliving.org)
But ultimately, she will need more than that, as time goes on.