I am writing this for a friend, but we should be able to respond quickly to details.
This book was popular around the mid 2000s. It was a horror/fantasy novel that featured a cartoonish, maybe graphic-novel-style cover, in vibrant colors with an old haunted-looking house on the front. The book itself was not a graphic novel. The story had something to do with a friend or neighbor of two siblings possibly being a werewolf (although I hesitate to add this detail because I worry it's misremembered). It is not one of the Goosebumps series, but the cover style reminds me of this series.
Thinking more on the book, I believe the title maybe had a number in it (potentially ten) and it potentially had the word "secret" or "confessions". I don't think the title had werewolf in it, and that may have been a subplot.
I would be so grateful if this sparks a memory from someone! Any details or tips that would improve my search would be much appreciated. Thank you!