One can intepret the things that the characters say differently but there's just too much evidence to back up Sydney and Sark's are full siblings!
1. the age (CALCULATE THIS: Syd was recruited at 19, worked for 7 yrs at SD-6 and a double agent for almost 2 yrs now. would probably be turning 28. David Anders just turned 22 and so could Sark. So subtracting Sark's age from Sydney's we get 6. Which is Sydney's age when Irina "died". Therefore, it's not impossible that she was pregnant.
2. Sark is not English, definitely! Becoz he said he was SENT to England to study. His youth could also be connected to Project Christmas (which Irina was stole from CIA thorugh spying Jack) in which while still in their childhood, exceptional children are being trained for the job. I mean, how else did Irina recruit and trained Sark?
3. As was ultimately revealed, Sark's loyalty is to Irina, not to Khasinau or even Sloane. Could it be becoz of a certain bond between them. (Not saying that Sark knows that he is her son.)
4.some say that Sark doesn't look like either Jack or Irina, I disagree. Look at Jack and Sark carefully and pay close notice of cheekbones and chin.
5. Anybody notice that Jack and Sark share the habit of making sarcastic or blunt remarks.