| Quote #4
If they dance any closer together, they won't have to go off in a corner to have sex. (12.64)
Here's our take: Clary probably wouldn't have cared if Simon were dancing with anyone else but Isabelle.
| Quote #5
"Let's not be hasty." Jace was actually grinning, the bastard. […] "[Simon's] cute like that. Look at his little pink nose." (13.115)
Even the third-person narrator thinks Jace is being a "bastard" for teasing Simon. And when the narrator thinks you're a jerk, well then you know you're on thin ice.
| Quote #6
"You'd be happy to leave him if it weren't for her," Isabelle said, managing to inject the single syllable word with enough venom to poison an elephant. (13.119)
By italicizing the pronoun there, we know that Isabelle really doesn't like Clary. So it looks like jealousy is a two-way street.