OP, I'm so sorry you and your family are going through this. I lost my mother to breast cancer, and my father of old age.
Watching a loved one die of cancer is very difficult.
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I know you weren't looking for advice, but if you want some, definitely reach deep and find the energy to be there for your family. If you can't be there in person. Call.
Know that while this is a horrible situation, memories of this will fade and you will be left with the loving memories of your mother. Free from all of this.