Did you notice in Endgame, Sark mentions that he was "sent to school in England" or something to that effect. Okay: here's an idea. Irina was pregnant with Jack's child when she "left" him/died/whatever. She then gave birth to a son ... guess who! Sent to school for his education, Sark is now pitted against his own sister in the subservise spy-games. Jack of course does not know about his son, and Sid thinks she's an only child. Who knows where it could go, but would definately make for a good cliff-hanger in the finales. P.s I know the S/S brother and sister thing is said already but I am adding more to it.