So far, the most pronounced negative reaction to Cassandra's arrival from Zander has been a drastic increase in the amount of baby talk. It is certainly tiring to me (consider posts like this and this), but is pretty textbook in terms of how an older sibling might react to a new baby.
We have a lot of exchanges like this:
Zander [comes running into room, half-crying]: "Bonky!"
Me:"Tell me with words."
Me:"You bonked your shoulder?"
Zander:"Bonky shoulder-y on floor-y. Need bandaid-y."I'm hoping it will pass sooner rather than later.