A bb gun is not a firearm. Apparently the sister's statement was correct. No real gun was found, nor anything illegal.
A BB gun isn't a firearm in Texas, and California.
However, this is Illinois.
Illinois has some very unique firearms laws.
Some BB guns, and even signal "guns" are covered under FOID in Illinois.
Heck, "nail" guns are covered by FOID in Illinois.
Cook County, especially Chicago tend be even more restrictive, belying the murder rate in Chitown.
First of all in order to buy a firearm in Il, you must first obtain a FOID.
FOID = Firearms Owners Identification Card
What is the definition of a firearm according to FOID?
"Firearm" means any device, by whatever name known, which is designed to expel a projectile or projectiles by the action of an explosion, expansion of gas or escape of gas; excluding however:
(1) any pneumatic gun, spring gun, paint ball gun or B-B gun which either expels a single globular projectile not exceeding .18 inch in diameter AND which has a maximum muzzle velocity of LESS than 700 feet per second or breakable paint balls containing washable marking colors;
(2) any device used exclusively for signalling or safety and required or recommended by the United States Coast Guard or the Interstate Commerce Commission;
(3) any device used exclusively for the firing of stud cartridges, explosive rivets or similar industrial ammunition; and
(4) an antique firearm (other than a machine-gun) which, although designed as a weapon, the Department of State Police finds by reason of the date of its manufacture, value, design, and other characteristics is primarily a collector's item and is not likely to be used as a weapon.
Here's the Illinois State Police Link concerning FOID questions:
Illinois State Police