I would add, just be prepared for the odd moment of sorrow to hit you when you're not expecting it. Possibly something that reminds you of any good memories (not clear if you have any) or feelings about your neices / nephews; maybe even memories of your mum's feelings.
It may or may not,I certainly wouldn't spend any more time worrying about it.
You won't feel sorrow for her death, because you lost her a long time ago. If there should be a feeling of sadness it should be for the loss of the life she didn't have because of her addiction. And sadness for the pain she caused others, especially your mum. Not wrong at all to feel how you do; better than pretending to feel something that you don't.
I had news today that my half sister has passed away. She was an an alcholic and because of that. her marrage ended, her two children would have nothing to do with her and she never got to meet her grandchildren . She made my mothers life hell and because of that mum also turned away from her, they never spoke after that and mum died a very sad woman. It was the drink that was to take her life but as of yet I have felt no sorrow for her death. I do feel so very sad of the life she made for herself she could not have been happy , is it wrong to feel like this