Who is Irina Derevko?
Irina Derevko (alias "Laura Bristow") was a deep cover Russian spy sent to seduce Jack and trick him into spilling secrets. They married, and she soon became pregnant. She considered terminating the pregnancy, but decided to carry Sydney to term, unprepared for the bond that would form between mother and daughter. When she was called back to the Soviet Union, she was forced to fake her own death in order to make a clean escape. Jack let Sydney think that her mother died without revealing any of the information about Irina actually being a Russian spy.

Why are Jack and Sydney so dysfunctional as a family?
As with any other family "under orders to lie to one another," the iciness between father and daughter has origins in Jack's emotional unavailability throughout Sydney's life.

Mostly, this is to do with Jack having led a life of deception long before Sydney would learn the truth. She grew up to view her father as an unyielding man of independent mind, with an igloo-like exterior; the ice wall had taken form long before they could begin to discuss their commonalities, or identify any professional likenesses.

There are numerous references to times in Sydney's childhood when she believed her father to be away on international business with Jennings Aerospace — in one case, for 6 months straight. But very recently, Sloane revealed to her that this was a period during which Jack was in solitary confinement in a federal penitentiary, following the FBI's investigation into collusion with his wife Laura Bristow and her KGB ties. Although his record was cleared later, it resulted in a breakdown and a period of heavy drinking.