We are the last ones

This is essentially a rewriting of city of lost souls.
Sebastian wants to burn the world down, but before he does that, he needs his sister. He needs her to approve, to love him and clap as the world turns to ashes. When, finally, his sister turns up in his secret dimension-moving apartment, he knows he has a chance. But, Clary loves Jace and sees her brother as a nothing more than a evil half-demon world-burner. To find the love of his sister, Sebastian must change. But, what if it's too late to change, or, worse, what if he can't change?