I am very familiar with the Chicago Megopolis. I live 90 miles Northwest of The Loop, 1 mile south of the IL/WI state line.
Chicago and its suburbs are 100 miles (north and south) by 60 miles (east and west). Campgrounds in that area are few and far between, but there are some. I'm assuming you want to be as close to your relative as you can so you won't have a long drive, or vice versa. What part of the Chicago Metro area is your niece, and from what direction will you be entering the area?