It was pulsating slightly, giving it the rather sinister look of some diseased internal organ.
Standing still and quiet in the gathering gloom, the creatures looked eerie and sinister.
Harry thought of the sinister winged horses he had seen on the night he had arrived and how Luna had said she could see them too.
She then bustled around the desk with it, smiling in a sinisterly sweet fashion.
’Well…’ said Moody, pushing back his bowler hat to reveal his sinisterly revolving magical eye.
There was a musical box that emitted a faintly sinister, tinkling tune when wound, and they all found themselves becoming curiously weak and sleepy, until Ginny had the sense to slam the lid shut; a heavy locket that none of them could open; a number of ancient seals; and, in a dusty box, an Order of Merlin, First Class, that had been awarded to Sirius’s grandfather for ’services to the Ministry’.
As Harry watched, one of the larger Catherine wheels seemed to decide that what it needed was more room to manoeuvre; it whirled towards Umbridge and Filch with a sinister ’wheeeeeeeeee’.
She was so short that this did not make a great deal of difference, but her fussy, simpering demeanour had given place to a hard fury that made her broad, flabby face look oddly sinister.
There was Mad-Eye Moody, looking quite as sinister with his bowler hat pulled low over his magical eye as he would have done without it, his gnarled hands clutching a long staff, his body wrapped in a voluminous travelling cloak.
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.