Who are your brothers and sisters? To a Christian or a member of another religion, often the fellow members of their religion are considered their brothers and sisters. Politics and other issues can also cause people to see one group as their family and one group as not; the battle between Republicans and Democrats is a very good example. But religion seems to be the biggest one.
Although it might make you feel warm and fuzzy inside, it divides the human race instead of bringing it together.
As an atheist, I see ALL humans as my brothers and sisters, even religious people.
Of course not all atheists think this way, and there are Christians and people from other religions that do see all humans as their metaphorical siblings. This is just an observation of what I have seen in my own circles. Because I grew up as a believer and was one for 24 years, I used to have hundreds of Christian people on my Facebook. Many of them would post about helping their Christian “brothers and sisters” and speak against abortionists, atheists, “false” Christians, liberals, democrats, etc. When they did post about other people in need, such as helping young girls in India who were rescued from sex trafficking, it was almost always with a catch: they were witnessing to those needy people so they would become “part of the family”. They couldn’t just help them with no other agenda- it had to be with the purpose of winning them over to their beliefs.
I saw how divisive this idea can be, and I felt the sting when I started to become public about my “non-conforming” ideas that many of my Christian friends did not like as my beliefs grew further from traditional church teachings. I wasn’t family anymore- and so they stopped treating me as family.
I think that any belief that divides the human race is harmful. When you see one group as your family and another as outsiders, or even perhaps enemies, then only hurt can come of it. I hope that you can see everyone as your family, even those that you don’t agree with.