I think your mother has behaved appallingly and if I am honest don't think I would ever forgive mine for doing something like this to me.
She didn't just not call you, she lied about speaking to you and made your brother think that you wouldn't come to the hospital.
She must have known that he would call you himself if she had told him the truth.
But even when she knew your Nanna was getting worse, she lied for a second time to still try and prevent you from seeing your Nanna for the last time while she was alive.
She's deliberately taken that opportunity from you and your Nanna.
In my book that is unforgivable.
I don't know if I would confront her or not. I doubt she will ever tell anybody the reason why she did it. But in your place I think I would find it very hard to forgive her and so even if I did speak to her, I don't think there would be anything she could say that would make me feel she had an acceptable reason to do what she did. The lying, more than anything, would make it impossible for me to forgive her. And if she continued to lie or tried to justify herself or play down what she did it would be impossible.
Of course, nobody really knows what they would do until they are in that position, and you are not me, but that was my gut feeling on how I might react.
I'm so sorry for your loss OP. And I am sorry your Mum has done this to you and your Nanna.