My mother is a very conservative Catholic Christian, and my aunt is a Presbyterian and works as a therapist for teenagers.
I thought that my mother would react badly, but so far she's been very supportive, more so than anyone else. I used to think that my aunt would take the news very well, seeing as she deals with adolescents every day, but I'm not so sure any more. People definitely react differently when it's someone they're related to.
Since your mother seems to be pretty easy to predict, I'd agree with everyone above and say to wait. At the same time, though, she may not really be in denial. I always thought that my mother was in extreme denial about anything to do with me, but it turns out that she noticed everything and just didn't really know how to talk to me about any of it.
You know your mother and aunt better than any of us, so ultimately, it's up to you to decide.