Nat, so sorry for your loss, and my condolances to you.
I think you were well within your rights to photograph your mother. But there are always instances where it's not possible to take a picture for one reason or another. Those are the times when we have to take the picture with our mind's eye, and store it in our memory. It was thoughtful of you to consider your sister's wishes as well, even if you think she was wrong.
Think of this, Nat ... if you *had* had your camera in the room at that moment, would you have perhaps been more preoccupied with getting the picture *just right* than you would have spent living there in that moment with her? As it turned out, that peaceful moment that the two of you shared was possibly more precious to your mom than if she had felt the intrusion of a camera right then.
And you will always have the image in your mind, to describe with words, to work through with other art forms ... I'm sure the image will find its way into your work in other forms. You may never share that exact moment with anyone else, but that inspiration can provide beauty over and over again. Use the color palette in your business cards. Put together a flower arrangement in shades of sunrise and ivory. Keep your ears open for a piece of music that could provide a soundtrack to the memory. Capture future sunrises glistening off of stones or buildings or other symbols of your mother's strength or personality. I know you don't have images of her final days, but you can symbolize her in so many ways.
Strength to you and your family.
~Kelly