Just playing Devil's Advocate here, but perhaps her mother figured Tori didn't care. Since she didn't bother to come home, as her brother did, when her father took ill.
Maybe her mother wanted to focus on the child who loved his father enough to choose his bedside over anything else, instead of the one informed of the stroke, yet still partying in Toronto and openly mocking the woman who's marriage she boasts about breaking up.
It's possible she's tired of Tori's immoral, selfish behavior that doubtless caused Aaron pain and shame, and with the death of her husband, just doesn't want to have to deal with that too.
I'm sorry for your loss and your pain, Tori, but you wouldn't have found out through email if you'd
been there. He didn't die suddenly. You had what, 4, 5 days? to manage, with all your wealth and resources, to find a flight from Toronto to LA. She has email, she's on the net - could it be she's never heard of Expedia?
Edited to add that I just found this, and Lainey says it better than I can:
http://www.laineygossip.com/ArticleList.aspx?ID=4118