Unless Jack met Sarah differently (and earlier) in the flash-sideaways timeline, I think Jack's wife is most likely somebody we have met before, given that her identity is hidden in the episode. It is possible Juliet and Jack's paths crossed in the flash-sideaways timeline at some point in their medical practice (or med school?) and got romantically involved, married, had David, etc. This would gell with what seems to be the nature of the alerations to the timeline, that there are clear differences to events even before the 815 flight.
Given that she and Jack did not work out in the original timeline, their relationship similiarly did not work out in the flash-sideaways timeline. She is away on business related to her medical practice - perhaps that brings her into an encounter with Sawyer?