trying to find a book that I read as a kid, if you're interested in helping thanks
All I remember (spoilers):
Kid and his friend go to the kids Uncle's house
Uncle lives in a mansion with a forest nearby
All of these weird things happen, and the kid must follow all these clues/quests
Ending reveals that its all a set of pre-designed games, and his COUSIN (girl) designed them - as the kid has won, he now designs the games for the next person
Uncle is revealed to be some sort of robot/machine (fake)
At some point, a young man (real estate developer) walks into the forest and is never seen again