Speaking as a militant atheist, I don't think that's true. Amongst my siblings, with whom I get on very well, I can identify the following: a staunch Baptist who's a pillar of the church; a trained minister who lost his faith in God but who has developed a fondness for the rituals of the Catholic Church; a pragmatic agnostic who as a young and less sceptical man smuggled Bibles into Soviet Russia; and a sister who's entirely unengaged with any kind of religious debate on the basis that it's less interesting than almost everything else. I have atheist friends, Christian friends, Muslim friends, Jewish friends and Hindu friends. (This isn't because I'm a particularly gregarious and tolerant guy - it just goes with being a Londoner.)
But sticking with the varied range of beliefs, currently and historically, among my siblings, I think it's worth asking why we get on well. It's because we share a view of the world that has to do with tolerance, kindness and the free, mutual expression of intellectual mockery - directed, often, at each other. We argue when we need to, we show utter lack of respect for each other's views when it amuses us to do so and we simply accept each other when that's what'll keep us all happy.
My Christian brother honestly believes that the fires of Hell await me, my (Catholic) wife and, unless something is done, my children. I think that he's kidding himself if he thinks he can avoid oblivion. However, we don't let any of this spoil dessert.