Hermione called, half sobbing as she and Harry were buffered by terrified guests: Harry seized her hand to make sure they weren’t separated as a streak of light whizzed over their heads, whether a protective charm or something more sinister he did not know — And then Ron was there.
Harry knew that it must be full of living creatures, but he wished they would all remain still and silent so that he could separate their innocent scurryings and prowlings from noises that might proclaim other, sinister movements.
He began to stride blindly around the room, opening empty drawers and picking up books only to replace them on the same piles, barely conscious of what he was doing, as random phrases from Rita’s article echoed in his head: An entire chapter to the whole Pot-ter-Dumbledore relationship… It’s been called unhealthy, even sinister… He dabbled in the Dark Arts himself in his youth… I’ve had access to a source most journalists would swap their wands for… "Lies!"
As they reached the end of this corridor, the whole castle shook, and Harry knew, as a gigantic vase blew off its plinth with explosive force, that it was in the grip of enchantments more sinister than those of the teachers and the Order.
There are no more uses of "sinister" in the book.
Show samples from other sources
They created a sinister plan to recruit terrorists.
In the movie, there was a sinister plot to kill people and harvest their organs.