Jin and Sun are secret lovers in flash sideways

In the sideways, Jin comes to Sun's room at the hotel to tell her he's going to take the watch to the restaurant. Both Jin and sun seem paranoid that someone might see them, so she invites him inside.

Once they're inside, Jin says that she is just there for a shopping trip. In the opposite of the buttoning of the sweater scene, Sun unbutton's her top button, then unbuttons the rest one by one until her sweater is off. Jin and Sun kiss and it cuts away.

The question we may never get about the flash sideways stories is what exactly changed in the past to make things turn out the way they did here. Since Sun and Jin's relationship is a secret and they're not married, it's likely that she never had an affair with Jae Lee.