Imagine: being Jace's sister and Jonathan's childhood friend, telling him you know who he really is and asking why he didn't come talk to you first.
You: I know who you are! you aren't Sebastian Verlac you are Jonathan Morgenstern!You got out and didn't come to me first!!?
Jonathan: Okay Yes I am sorry I didn't know what you would say.
You: I love you Jonathan! I wouldn't have told anybody.