EnderPL
I feel like this can only lead to an all out war with the responsible House. And although the Lanisters seem to be the ones who are trying to kill, either Barbey, Bri or both, I actually think that it is another house, because of the poison that was used and the fact that Tywin Lanister is too smart to do something like this. Olenna Tyrell is famous for using poison and even though the author didn't say that the girl turned purple, I think that an extinct House like Tyrell would try to do something like this to stir up things in Westeros and attempt a comeback of sorts. Also it is possible that Varys is trying to weaken the most powerful families so that there is more stability in the realm. The rebellion just happened and Varys always talks about how he does everything for the Realm, so I think that he is trying to even the fields, even though Robert is not showing anything kingly even at the start of his reign, he is the king and another Rebellion wouldn't make things easier for the stability of the realm itself. He would probably advise the king to stand aside if the Lanisters and Dustins start a war amongst themselves and would explain it as "A way to weaken the Houses and put the Royal (Baratheon) House back on top with a big lead.