The quote from the summary is on page 200 of City of Ashes, btw :]
He had done it a lot, when he was younger. It was the easiest way to get rid of pain. He would just chant, monotone, over and over, until the word ceased to mean anything. It was that way he learned that nothing in the world was permanent- that anything could be erased if you just tried.
He didn't do it anymore. It was stupid and childish. But lying in bed, staring at the ceiling, Jace can't help it.
"Sister sister sister sister sister, Clary is my sister sister sister sister sister."
He chants that word over and over, but this one is different. By chanting it, he doesn't make it weaker- he makes it stronger. Instead of ceasing to mean anything, it is now the most prominent word in his dictionary. He can't escape it- he can't make it disappear. It just hangs there, in front of him, taunting him, and now Jace can't even stand to say the word, can't stand the way it sounds.
Sister sister sister sister sister, Clary is my sister sister sister sister sister.