My Mother's Grief

My brother died 20 years ago. My mother went on to do amazing things to educate and help others with grief. She teaches about understanding anticipatory grief, the need for hospice for children and talking through the process of grieving. But my sister is forever encased in that. How do i broach this? It's sacrilege in our family to question grief. Every time i've tried, the conversation turns into one in which I'm reassuring her that she's not a terrible mother. Any advice? Sugars, i am at a loss. Signed, forever grieving.