What's the Name of That Book??? discussion

I'm trying to identify a children's or YA novel that I borrowed from an Australian public library around 2010 (possibly published earlier).

The main character is a young boy whose sister is missing or dead. He believes that a large shadow/insect-like monster took her, so he spends the book investigating what happened. By the end, it's revealed (or strongly implied) that the monster never existed—it was a psychological manifestation that helped him cope with his sister's disappearance or death.

I vaguely remember the cover showing the boy with his shadow turning into a giant insect or monster, but I may be remembering the cover incorrectly.

Does anyone recognise this book?

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