Before reading Bree's story, I believed that Jane know something but wait until it was too late to intervene. Now that visit put thing on other perspective… Was a decision form the Volturi (and Aro) or an idea of Jane?
Aro always pretends to be Carlisle’s friend and we know that Jane hates Bella and may considers Alice and Edward as a threat for her position in the Volturi’s guard.
On the other hand, Edward, when he went to Italy, gave Aro some clues on how powerful the Cullen’s coven is, and how Carlisle has succeeded on his attempt of living without killing human. At this point, may Aro have been so jealous to let this little army split the Cullen’s coven, broke their power, their deviant way of life and hope that some of them will join the Volturi?
And for Diego and Bree… I don’t believe they will have long live if Jane know they spy her...