Law enforcement sources tell TMZ ... Jordan Miller -- who's a year younger than Jason -- was walking across the street in Fort Lee Wednesday afternoon, when a NJ Transit bus barreled into him while making a left turn, according to a preliminary police account.

We're told Jordan was rushed to a local hospital, where he was initially listed in serious condition, but then took a turn for the worse and was later pronounced dead.

Fort Lee PD detectives are investigating the circumstances surrounding the incident to determine if it was an accident or a crime. Jordan was a customer service rep at the Port Authority Bus Terminal in Midtown Manhattan. A Port Authority rep confirmed to us Jordan is, in fact, the sibling of Jason ... who apparently was friendly with folks in the agency.

As you probably know, Jason is a big-time Hollywood actor with a laundry list of film credits -- including "The Lost Boys," "Sleepers," "Geronimo: An American Legend," "Your Friends & Neighbors," "Narc," "The Losers," "The Alamo," and "Speed 2: Cruise Control." Jason has also appeared on some TV shows, such as "Entourage" and "Saturday Night Live" as host.

We reached out to Jason's camp ... they had no comment. Jordan was 56.