Here is Clary, being oblivious to the most obvious thing ever. I really should be making this a das_mervin style sporking count. Seriously, Clary has been obsessing over Jace, even towards the end of City Of Bones days after it was revealed that Jace was her brother. Yet, if you disregard that, Jace has been acting like a jealous ass-hat ever since he found out that Simon and Clary were ‘together’.
Anyone could have seen that Jace still has romantic feelings for Clary. Actually, it’s pretty obvious that Clary and Jace really don’t care that they are siblings, at all. Sibling incest really never has put a damper on either of their feelings, and I really wish that Cassandra Clare would stop pretending that this is a huge conflict, it really isn’t. It doesn’t seem to bother either Clary or Jace that they are related, so it kills any conflict that could have been caused by the reveal at the end of the first book. Your ‘conflict’ isn’t a conflict if your characters don’t consider it a conflict.