Fans of the Arrow, but this is largely based upon one of the biggest misunderstandings in the world of comics.
The Black Lightning season 4 episode "The Book Of Reconstruction: Chapter Three" saw both Dr. Lynn Stewart and Jefferson Pierce continuing therapy, both as a couple and alone, to work out the conflict in their marriage. In Lynn's individual session, she had a breakthrough regarding how she had always seen herself as an outsider within her own family, due to everyone else in her family having served in the military; something Lynn was discouraged from doing, due to her being a skilled enough student to pay her way through college with scholarships. In discussing this, Lynn mentioned having been jealous of her brother John, who enlisted in the US Army and shared a bond with her parents that she never had.
Many fans assumed this was confirmation of an older fan theory that a Green Lantern ring.
While it's a fun idea, drawing a connection between Lynn Stewart and John Diggle is based on one of the biggest misconceptions in DC Comics history; that John Stewart and Lynn Stewart are siblings. While Black Lightning creator Tony Isabella did give Lynn the last name Stewart with the intention of maybe connecting her to the Green Lantern character in a future story, he never had the chance to do so in any Black Lightning comic he wrote, nor did any other Black Lightning or Green Lantern writer ever make the connection. The only setting where Lynn Stewart and John Stewart were confirmed to be brother and sister was in the reality of the Young Justice animated series.
Beyond that, the fine details of Lynn Stewart and John Diggle's lives in the Arrowverse don't line up. Diggle never made any mention of having a sister; a curious omission given how highly he valued his family, particularly his younger brother Andy. Another conflict is that Lynn spoke of her family all having served in the US Army. While Diggle was a Master Sergeant in the US Army, his stepfather was a General in the Marines; a distinction someone from a military family would be certain to make.
It's been suggested that all of these details could have been changed in the wake of Crisis on Infinite Earths and that reality could have been rewritten so that Lynn Stewart and John Diggle were siblings, in the same way that Diggle acquired twin children in the final episode of Arrow. However, it seems highly unlikely the writers would go through this much effort merely to tease an indirect connection to Green Lantern. Additionally, while David Ramsey is supposed to be making cameos as John Diggle in most of the Arrowverse series this year, he is not reported as appearing in Black Lightning, which would seem a certainty if he were meant to be Lynn Stewart's brother.