He is unhappy.

I could bear what is called disgrace, my lord--I shudder to say it--I could sin against heaven; but I dare not do what would make him despise me.' She was trembling violently; yet the nobleman, in his surprise, could not forbear from asking who this person might be, whose influence on her righteous actions was so strong.
'It is my brother, my lord,' she said.
Still more astonished, 'Your brother!' the Duke exclaimed.

'My dearest lady, I would not wound you; but is not this a delusion?
We are so placed that we must speak plainly.

Your brother I have reason to feel sure is quite unworthy of you.' 'Unworthy?
My brother Evan?
Oh! he is noble, he is the best of men!' 'And how, between yesterday and to-day, has he changed you ?' 'It is that yesterday I did not know him, and to-day I do.' Her brother, a common tradesman, a man guilty of forgery and the utmost baseness--all but kicked out of the house! The Duke was too delicate to press her further.

Moreover, Caroline had emphasized the 'yesterday' and 'to-day,' showing that the interval which had darkened Evan to everybody else, had illumined him to her.