Stop right there. Your dear brother is already presupposing things that he cannot account for, explain or justify, given his atheistic world-view. He is adopting a presupposition contrary to the conclusion he wants to argue; namely, that there is no God.
Already, when he says, "should", or (later) uses the word, "hypocrite" he presupposes some sort of absolute standards of morality or reason that are universal and prescriptive in nature, the very things that are precluded by his own world view. His prescriptions do not make any sense unless it is objectively wrong, for example, to redefine the word "liberty", marry big government and big religion, or be a hypocrite.
But how does he justify his notion that some things are good and some things are evil when in the atheistic world-view everything just amounts to different combinations of matter in motion? What sense does it make to think that matter in motion is not behaving as it 'ought" to?
Your brother's atheism cannot account for his moral umbrage. He is not in any position to explain the objective and unchanging nature of moral notions like good or evil because his atomic makeup as opposed to someone else's atomic makeup ultimately reduces to mere relativistic preferences for thinking,
And because your brother is a finite, ever changing collection of molecules and electrochemical reactions he is also in no position to pronounce on what is universally true even in a descriptive sense because the aforesaid electro-chemical reactions that make up your brother are limited in the scope of their use and experiences.
In short, his arguments and claims are self-vitiating and incoherent.
Very well put. It is a point I am constantly making to Atheists. They rise up out of a sea of Christianity, and think their current moral opinions are independent of the Christian influence which molded and shaped them.
All the Atheists today have moral opinions heavily influenced by Christianity, but they are completely oblivious to this fact. Remove the Christian contamination from their moral perspective, and they will have the same moral convictions as Joseph Stalin. This experiment has been run numerous times. Atheism left to it's own devices is a recipe for mass murder.