…Michael Wayland was dead?
Many people have asked this, probably because it is indicated elsewhere that parabatai can tell when the other one is dead, though it does depend on circumstances and the strength of the bond.
In City of Bones and subsequent books it is made clear that the Lightwoods live “in exile” after having been punished by the Clave for their role in the Uprising. There are a few things that sever parabatai bonds. Death. Someone becoming an Iron Sister or Silent Brother. Someone becoming a Downworlder or mundane. And exile.
As an exile, Robert would not have expected to feel any connection to his parabatai and would not have known he was dead.
Robert himself says it in City of Heavenly Fire.
Alec: “You didn’t seem to miss [Michael] much, or mind that he was dead.”
“I didn’t believe he was dead,” said Robert. “I know that must seem hard to imagine; our bond had been severed by the sentence of exile passed down by the Clave, but even before that, we had grown apart.”