So in the episode where Clary goes through that portal in the woods to an AU, she's with her father Valentine and her mother. It's pretty obvious she's their only kid. And she's dating Jace in this universe as well. The only thing different in the universe is there's no shadowhunters.
I understand Clary and Jace might not be thinking clearly after Valentine drops that little bomb about being Jace's father, but SERIOUSLY? Can't Clary use rational thinking and go "If Valentine is Jace's father, why wasn't he my brother in the AU"
It would have been such a great way to just get that stupid incest crap out of the way. The show is already better than the books, I wouldn't care if they dropped the source material like a hot potato and just did their own thing.