Hey, Lisa. In all honesty, I became an atheist around the same time as yourself. I haven't told my family, beyond my sister, but I think it's implied that I don't believe what everyone else believes. But to address your question about revealing your position to your mother, I wouldn't right off. Unless she asks or you feel that her religious position is imposing on your life (which I assume you'd be against), then reveal that to her.
This is shocking and saddening news. I'm truly sorry to hear about your loss.
My sister passed away on Aug 31, 2012. At one point, when I was at a particularly weak point, I sobbed into my brother's arms. And he, believing me to still be a believer in fairy tales, tried to comfort me with the words "we do not mourn as they do." Meaning everyone around us, "those who have no hope." I darned near punched him out. We do not mourn as they do. Of all the arrogant exaltations of pathetic denial...
I mourn as a human being does, a human being who has suffered loss and recognizes that time only goes in one direction. It is a sad and beautiful and REAL thing. The Christian "hope" robs us of the reality of loss and mourning. We lost a beautiful and wonderful person, and he's treating it like "see ya later!"
No. No we won't.
And that's sad. And real.
My sister now lives in the memory of everyone who ever loved her. She would have appreciated the honesty if she had come to the understanding about faith that I have. I truly miss her. I truly mourn her. As others do. As others should.
My condolences on the loss of your mom.