Re: What kind of reaction would I get if....
Because yor mum is religious, you can probly expect a lot of emotional appeals from her and this would be tough to counter.
As for the rest of them even though they may not be very religious, be prepared to hit a raw nerve when you announce. Because it may be a sensitive matter to one of them as it is such a fundamental belief/non-belief.
So unless you are sure, I dont feel you need to announce it...unless if they notice why you used to pray/fast previously and now why you dont and you need to explain this to them.