Since there is no Uncle'ing on StackExchange, I figured the Parenting site is the closest match to my dilemma.
First some backstory. My sister and I were raised in a mildly conservative (middle of the road) Jewish family. At an early age, I realized I was agnostic/atheistic. My parents always said it was a phase and I would grow out of it. However, over time, my opinion only solidified. This of course caused conflict between my parents and me. As the years past, they have come to live with my choice. My sister's participation dwindled to almost none, when she started her own life.
Now that she has kids, she is more participatory in religion again. She goes to synagogue and sends them to Hebrew and Sunday school. I asked her why, she said it was good for the kids to learn about the religion and culture. To be honest, I don't even know if she even believes in God or religion; she never talks about it. And I never ask, as it has zero importance to me.
My nephew is having his Bar-Mitzvah (the Jewish rite of manhood) later this year. I was asked to participate and I agreed. Naturally, I want to see my family, as I don't see them very often. However, as my sister's kids get older, sooner or later they will ask me about my beliefs and religion. Quite possibly at a religious event that is coming up in a few months.
I will not lie about my beliefs, or lack thereof. I am proud of the fact that I have turned away from theistic beliefs and religion. However, I can see potential conflict with my family. I am tired of hearing my parents say, why can't you respect our religion, when they have no respect for my lack of religion. I see this conflict more with my parents, but I am afraid to hurt my sister's feelings as well.
When the situation finally rears its head, how do I talk about my beliefs to my nephew and niece in a way that won't cause (or at least minimize) conflict with the rest of my family without compromising my own beliefs?