People have so much hate for TMI because of the incest plot completely forgetting that, that's the whole point.
Jace and Clary were just two teenagers that were manipulated by a grown ass man to make them think that they were horrible people so that they would help him. Valentine experimented on them and psychologically tortured them its a terrible situation for anyone to be in they were being abused and people need to realize that.
It's not as if they found out the were siblings and just decided they didn't care. No they did care and they broke up Clary even tried to move on by dating Simon but she just couldn't suppress her feelings for Jace which made her feel like she was disgusting and by proxy us the reader, which was exactly what Valentine wanted Clary to feel like.
And then you see Jace who believes he is part demon and he feels like a monster for loving Clary and he feels like he is destroying her. He thinks he's irredeemable which is EXACTLY WHAT VALENTINE WANTED. He wanted a son who would do horrible things because he thought it didn't matter cause he was damned if he did damned if he didn't anyway right.



















