How does Caesar react to Cimber's pleading to recall his brother from banishment?

The pleading is just a ruse for the conspirators to encircle Caesar, hemming him in and shutting others out, including others who might try to give Caesar help against the assassins. Cimber should know that it is pointless to beg Caesar to rescind his brother's banishment, since Caesar is expected to be dead within a matter of minutes. Cimber's pleas, which are then echoed by many of the assassins, only seem to bring out Caesar's enormous egotism. He is already behaving like the autocrat he would have become if he had managed to stay alive. It would seem that Shakespeare waited until this moment to demonstrate that Caesar was supremely ambitious and deserved to be assassinated. Up to this point Caesar has been pretending to be modest and democratic. Casca describes how he behaved in front of the crowd when Antony offered him something like a crown in an obviously staged performance for the populace.

Marry, before he fell down, when he perceived the
common herd was glad he refused the crown, he plucked
me ope his doublet and offered them his throat to cut.
An I had been a man of any occupation, if I would not
have taken him at a word, I would I might go to hell
among the rogues. And so he fell. When he came to himself
again, he said, if he had done or said any thing amiss,
he desired their worships to think it was his infirmity.   (I.2)

This is a far cry from the way Caesar expresses himself on the Ides of March when he is expecting to be officially crowned by the Roman senate.

I could be well moved, if I were as you;
If I could pray to move, prayers would move me;
But I am constant as the northern star,
Of whose true-fix'd and resting quality
There is no fellow in the firmament.
The skies are painted with unnumber'd sparks;
They are all fire and every one doth shine;
But there's but one in all doth hold his place.
So in the world, 'tis furnish'd well with men,
And men are flesh and blood, and apprehensive;
Yet in the number I do know but one
That unassailable holds on his rank,
Unshaked of motion; and that I am he,
Let me a little show it, even in this;
That I was constant Cimber should be banish'd,
And constant do remain to keep him so.    III.1

The Soothsayer warned him to beware the Ides of March. The warning might have been intended as a caution against revealing his hubris before he had received the crown. Caesar does not seem like a man who will be satisfied with merely becoming king. He would go on to become emperor and then to become a god. (His successor Octavius became an emperor and a god.) Julius Caesar's references to the stars show that he is in the habit of looking up at them and choosing his place among the gods and demigods. Caesar would like to be the pole star, which remains in place while all the other stars circle around it. Even Brutus can see that his friend has the potential to become a terrible tyrant. The audience can see this too and can understand why there are so many conspirators and why they stab their victim so many times. Shakespeare has kept Caesar's ego under wraps, so to speak, because it is more effective dramatically to reveal it to the world at this high point in the play. It is strikingly ironic that Caesar should think he is on top of the world only moments before he is slaughtered and turned, to use Antony's expression, into a "bleeding piece of earth."