When was this? Where was this? What were the circumstances? Why we LEOs in his home in the first place? I don't know the situation, but your brevity does not impart a favorable situation for your ex-BIL. First thing in my mind was that he was arrested on a warrant serve and the LEOs rooted through his stuff to tack on to his sentence.
He violated the Oak Park, Illinios gun ban.
Someone had stolen his license plate while he was coaching a little league team. They robbed a bank. The police came to his house and asked his dimwit wife if he owned a gun. She gave them the gun. They locked him up.
He was sentenced to community service and time served so he went back to coaching the little league team.