If it’s worth saying, but not worth its own post, here's a place to put it.
If you are new to LessWrong, here's the place to introduce yourself. Personal stories, anecdotes, or just general comments on how you found us and what you hope to get from the site and community are invited. This is also the place to discuss feature requests and other ideas you have for the site, if you don't want to write a full top-level post.
If you're new to the community, you can start reading the Highlights from the Sequences, a collection of posts about the core ideas of LessWrong.
If you want to explore the community more, I recommend reading the Library, checking recent Curated posts, seeing if there are any meetups in your area, and checking out the Getting Started section of the LessWrong FAQ. If you want to orient to the content on the site, you can also check out the Concepts section.
The Open Thread tag is here. The Open Thread sequence is here.
Theism is a symptom of epistemic deficiency. Atheism follows from epistemic sufficiency, but not all atheists are rational or sane. The epistemically virtuous do not believe on insufficient evidence, nor ignore or groundlessly dismiss evidence relevant to beliefs they hold.
That goes for both of you. The Litany of Tarsky is the correct attitude for a rationalist, and it's about not thumbing the scales. If your brother were sane (to rationalist standards), he would not hold such a belief, given the state of readily available evidence. If he hasn't figured this out, it's either because he's put his thumb on the scales or refuses to look. Organized religions (that have survived) teach their adherents not to look (ironically), and that it is virtuous to thumb the scales (faith), and that is something they have in common with cults, although not always to the same degree. These tactics are dark arts—symmetric weapons, that can promote any other beliefs (false or otherwise) just as easily.
If you feel like talking to him about it, but don't want it to devolve into a debate, Street Epistemology is a pretty good approach. It can help dislodge irrational beliefs without attacking them directly, by instead promoting better epistemics (by Socratically poking holes in bad epistemics).
To answer your direct question, I think Privileging the Hypothesis is pretty relevant. Einstein's Arrogance goes into more detail about the same key rationality concept of locating the hypothesis.