Why did he exile his older brother??

For me, having read the novel, the scene makes it a little clearer that the reason why he was exiled was:

first, to save his life—because what he did constituted treason as a member of the cult, knowing that his leader and their cult were searching everywhere for that beggar and he had discovered them, yet he chose to hide it; he failed in his duties and his role as right-hand man

Second, because the leader believes he is going to die and needs someone with enough power who already understands how the cult operates to succeed him—and who is better suited for that role than his sweet brother.

Third, everyone in the cult knows that betrayal is repaid with murder, but no one learn how kindness is repaid; so I think one of the reasons he didn’t kill him was also to repay the emotional debt he feels toward his older brother for everything he did for him when he was a child, and that his final act of mercy toward him—because, let me remind you, when he exterminated his brothers and sisters as well as his dad, he chose to spare his older brother, knowing that one of the reasons he was able to survive for so long—even if he doesn’t want to admit it openly—is thanks to his older brother

So for me it the best outcome for the older brother ╮(￣▽￣)╭

Wow! thank youu very much!! Now everything makes sense.

I just want to ask you what happens to the older brother if ML survive in the future (which he will probably)

Well basically he still exiles in the end