Post-ROTJ, so they know they're siblings. Something goes wrong while they're chasing stray Imperials and they're forced to spend a night somewhere really cold, and oh no! the only way to keep warm is to be in close contact (or other plot reason for them to be forced to cuddle you can come up with). This brings up the attraction they felt before they knew they were siblings and they find they can't resist. Can either be an angsty one-time thing or lead to OT3 with Han.