I have different views on this. I started off thinking that Sark is the child of Irina & William Vaughn, because he resembles Vaughn so much. But, if William Vaughn died when Michael was 8, that would be 1976. If JJ is using David Anders actual age, like he is with Michael Vartan, then Sark would not have been born until 1981.
My new theory is that he's Irina & Jack's son. Sydney is 28, which means she was born in 1975. Her mother "died" when she was 6 -- in 1981. What if Irina was pregnant at the time? And Jack didn't know? I definitely see Sark as Irina's child though.