Because it's supposedly a "family" oriented thing that is supposed to teach "morals" and "tolerance." It hurts because there's no way to not offend someone simply for having no belief, as if I don't believe just to piss them off.
It hurts because I see the fear in her eyes when we get close to the subject of my "soul" after my body dies and she can't imagine what "Heaven" would be without her sister yet she doesn't understand the inconsistency of a God who wouldn't let me in.
It hurts because she'll never listen to my story, she'll never hear how long and hard the path to atheism took and how no matter how I looked at it, I couldn't bend the evidence in favor enough to let me have a leap of faith; the gap was too wide. It hurts because she'll never hear it, not because she can't, but because she won't. She simply won't listen.