Word of the wreck was spread Saturday in a FB public community page specific to Stratford -- which highlights classic cars and expos in and around the Canadian region within Ontario ... where Bieber has family ties. A photo of the aftermath was posted, with what seems to be a page administrator asking if anyone in the community could ID who it belonged to.

A guy named Brandan Steven, who also appears to be related to Justin, came forward to claim he was in the accident depicted, and that Kathy Bieber -- JB's grandma on his dad Jeremy's side -- was the owner of the convertible, and that she too was inside.

Of course, folks on the page asked for more details about what exactly had happened here -- to which Brandan replied, "we were at a dead stop as a car was turning left a couple cars in front of us. Then a distracted driver rear ended us they say she had to be doing 70."

He goes on to say, "Wasn't good had to pull my grandmother out of car. could have been very bad if we were in there for even another 5 seconds as the doors were binded shut."

Within the comments of this post, people actually tagged Jeremy Bieber's FB page ... as well as Kathy herself, who has photos posing with the car. Of course, this is also the same Kathy with whom Justin has posed in the past ... specifically, when he was much younger.

There aren't a whole lot of public photos of him with Kathy and her husband, George ... who's Jeremy's biological father, but they exist and have been well-documented online. Kathy is Jeremy's stepmother, and thus ... Justin's step-grandmother.