What would you do if your brother did not believe in God?

[Zargon]

Since you are siblings, he no doubt went through the same religious education you did.

So he knows everything about your religion that you do. He just thinks it's implausible.

Think about it. You believe in all kinds of wildly implausible things as a Catholic. How likely would you be to believe any of the miracles attributed to Jesus if they were said a guy named Dave who's a carpenter from Philadelphia? How likely would be to believe that Dave was raised from the dead? How likely would you be to believe that Abe Lincoln parted the Mississippi and led the slaves out of Missouri to freedom in Illinois?

Your brother's decision that he does not believe in God is something you should have expected, because a belief in God and particularly in Catholic teachings runs counter to common sense.