I've watched enough bad soap operas to see the warning signs about this sort of revelation coming. I'm getting the bad feeling Prometheus is going to turn out to be Oliver's half-brother via Robert. I've felt all along that this whole being Justin Clayborne's son angle is misdirection.
My stupid theory is that Prometheus blames Oliver for Robert's death. This season keeps implying that Prometheus ties back to the first season of the show if not to the very first episode. They've made a big deal about a lot of Adrian Chase's background has been hard to dig up. Even though his parents are supposedly named Justin Claybourne and Amanda Westfield he used the name Simon Morrison before becoming Adrian Chase.
I'm thinking Robert (or maybe even Moira) paid off Justin's mother with a considerable about of money and she reinvented herself with the Amanda Westfield identity (I think she was a Morrison).
Did Flashpoint reboot Talia, as she seems to be different person now!