The brothers Knightley are both agents of change for Emma and especially Mr. John Knightley as the dawning of Emmas' self-realization.

From Ch 13:

"Yes," said Mr. John Knightley presently, with some slyness, "he seems to have a great deal of good-will towards you."

"Me!" she replied with a smile of astonishment, "are you imagining me to be Mr. Elton's object?"

"Such an imagination has crossed me, I own, Emma; and if it never occurred to you before, you may as well take it into consideration now."

"Mr. Elton in love with me! What an idea!"

And again from Ch 16:

".....To Mr. John Knightley was she indebted for her first idea on the subject, for the first start of its possibility. There was no denying that those brothers had penetration. She remembered what Mr. Knightley had once said to her about Mr. Elton, the caution he had given, the conviction he had professed that Mr. Elton would never marry indiscreetly; and blushed to think how much truer a knowledge of his character had been there shewn than any she had reached herself. It was dreadfully mortifying; but Mr. Elton was proving himself, in many respects, the very reverse of what she had meant and believed him; proud, assuming, conceited; very full of his own claims, and little concerned about the feelings of others....."

Finally Emma begins to think of Harriet as an independent entity!