How do I help my mother, who is caring for an elderly lady, from losing her sanity?

Oma came to live with us 2.5 years ago. She is a widow of 2 years, and was diagnosed with dementia 5 years ago. The dementia has progressively gotten worse as the time without my Opa has gotten on. My mother and I are her primary care givers, and we take the brunt of the verbal abuse. I'm a supervisor in a doctor's office, and get all the medical advice I need, but I don't know how to help my mother deal. My Oma takes much of her frustrations out on my mother, accusing her of hurtful things. As much as I try to tell mom that its not her fault, its not her mother saying these words... truth is, it is her mom. Oma was my mother's super-woman and seeing the way that she is regressing is killing my mother. How do I help my mother, and preserve my mother who is slowly losing her sanity?