Two weeks ago my sister found out that I was an atheist. Things went better than expected, but I didn't talk to her face to face about it until yesterday. I walked to the UCSD campus, and we had an awkward but perfectly amicable discussion about my situation over lunch.
We didn't talk at all about my actual reasons for rejecting Christianity and becoming an atheist, because I didn't want things to get too heated. She did say she was surprised that I hadn't simply declared myself agnostic, which I cleared up pretty easily by explaining the technical definitions of atheism and agnosticism. Atheism relates to belief, while agnosticism relates to knowledge. Most atheists (myself included) don't claim to know that no gods exist, and thus would be classified as agnostic atheists.
My sister said that while she was sad that I had left the faith, she wasn't going to condemn me for it. She respects the atheist position, and acknowledged that there are a lot of dumb Christians out there, but said she didn't like people that call Christians in general stupid. I told her I agree: intelligence isn't really something that factors into religious belief one way or another, because people tend to keep their intellectual pursuits separate from their religion. She was also surprised and dismayed to learn that a couple of people have already tried to tell me I was never really a Christian—as if they could know better than I do what goes on in my own head.
Mostly, though, we talked about how I plan to tell my parents that I've stopped believing. Both of them would certainly be very upset. My mom would probably get pretty emotional, while my dad could get angry and defensive if we started getting into specifics. We discussed several potential options: a slow phase-out starting with a decision to stop attending church, an indirect solution like a bedside letter, or a more forthright across-the-table talk. In any case, she agreed to be there for the big reveal if need be.
Overall I think it went very well. My sister was very kind and understanding, although I'm a bit concerned that she might be less supportive if she knew that my attitude toward Christianity was one of strong distaste rather than mere disbelief. For now, the important thing is that our relationship isn't at all strained or defined by our beliefs or lack thereof. We can still talk and laugh about that annoying professor or the latest episode of that hilarious TV show without our differences getting in the way.