Looking for a YA horror book (?), possibly British or Australian, 2000s or earlier, about a girl whose younger brother starts acting strangely. It's split into three sections: in the first he is acting similar to a person possessed by a demon (and a priest is called to exorcise him). In the second section he calms down and starts to become obsessed with the stars, implying that he is an alien. In the third section he calms down a little more, and the novel ends with some sort of climax/sacrifice? after which he returns to normal. The cover had a very pale ghostly looking boy on it, and I think the title was a one-word, strange sounding (making starting with a?) name that the boy called himself (I think based off a constellation). I read this book in the early 2010s as a child and was very disturbed but fascinated with it.