In the third place, the sister being alive would somewhat account for Jonathan’s lack of remorse or trauma after her supposed death all those years ago, and if that story didn’t come up during the trial, then Jonathan might indeed have been later acquitted.
In the first place, it would make Jonathan look shitty for again lying about his childhood trauma.
Had Sylvia actually been Jonathan’s sister, that would have thrown a really weird wrench into the works, and it’s hard to consider exactly what ramifications it would have.
In the second place, it would have indeed made the mom out to be a total liar during her talk with Grace.