They're too close in age
The cliffhanger at the end of episode 4 of Line of Duty's sixth season has the whole nation scrambling through old series, piecing together tangential plot lines, and trying to predict who this mystery of Jo Davidson's might be.
The scripting of the final scene was delicate, if slightly annoying. Steve could have just... said the name of the person, but that would kill the suspense! And if there's one thing we know about Line of Duty, it's that they love to keep us guessing in the time between episodes.
So who is this nominal, known to the police, who is a blood relative of Jo Davidson's? Many viewers seem convinced that it's John Corbett, from series 5. Corbett is the biological son of Anne-Marie McGillis, an old informant to Ted Hastings in Northern Ireland. McGillis was killed by the IRA in 1989, at which point Corbett was sent to Liverpool to be raised by his aunt and uncle, hence the accent.
Corbett was killed off in series 5, when the OCG finally had enough of his nonsense and Ryan slit his throat. So he won't be coming back, but going by Hastings' reaction to Steve's revelation, and the photo of what appears to be Jo's mother in her bedroom,
fans seem convinced he is Jo's brother.
It seems like a sound enough theory until you dig a little deeper. According to Lineofduty.fandom.com, where all the official information on characters' backstories is documented, John Corbett was born on June 3rd 1979, while Jo Davidson was born on April 22nd the same year.
You don't need to be a biologist to do the maths and work out that they can't be siblings.
Furthermore, Ted was close to John's mother, and knew about her son, but there was never any mention of a daughter.
So despite the vague similarities between McGillis and the unidentified woman in the photograph in Jo's bedroom, it's extremely unlikely that Jo and John are related.
