After they get off the ship, they have concocted a story to protect Aaron (who was not on the flight manifest) from being put into someone else's custody, and they say that Kate was pregnant while on the plane. She could have been five months pregnant without anyone knowing it, including the marshall. Remember she was a fugitive and noone was really keeping track of her physical condition. After 3-4 months on the island a baby could have been born to Kate, and she could claim the father's identity was unknown because she slept with a whole army of barflys in Australia. The time difference off the island could explain why he appears a few months older. Since she was not jailed until the trial started, she would be allowed to keep the baby. Nobody would really be suspect of a group of rock star survivors telling a big fib. Aaron's true mother is just part of the story. Jack may disagree with the lie, but he's not going to blow her cover.
This is just one possibility. Farfetched, maybe, but then again so is the whole premise of the show. Don't stress Cindi, it's just television.














