Did they ever specify a timeframe? The lack of information leads me to believe that the writers haven't figured out their backstory yet. I'm probanly wrong but I get the sense that Dex did a tour in Aghanistan, and during that time her parents left Ansel in a home(?) and left. When Dex was discharged she had him released with her as guardian (Ansel still obviously a minor) and that's how they have been since Ansel moved out. There must have been neighborhood gossip about the Parios' putting their son in a home and skipping town. Maybe fraud, income tax violations, charity theft were in the future for them. Who knows, almost certainly NOT the writers otherwise they would not be delaying the reveal so late in the series.
I'd like to go on record that Grey going undercover and Dex playing a wealthy buyer are total catastrophes in-waiting. Grey may have the skills to portray a criminal, after all, he's a convicted felon with a shady past but Dex portraying a woman of substance? Puleeze, the woman is just a crafty vet with PTSD, AND BASICALLY ZERO posh. She's a tomboy and probably can't pull off what they want from her. Anyway, I think Hoffman is asking too much from Grey and if something happens to Grey, Hoffman is going to have Hell on his hands. Speaking of Hell, YAY, EVE!!!!! Max, played by Inbar Lavi, who played Eve on "Lucifer", is a welcome sight and I'm glad she's getting work. Just be available to play Eve in season 5 of "Lucifer", please.