They double up because the end of the Emperor wasn't the end of the war entirely and supplies are still limited. People know they are siblings now and think it will be less awkward for Luke and Leia because at least they know each other which is more than can be said for some. If only. They have been trying to fight at the pull between them but now their efforts lie in tatters in the tiny spaces between them as their knees press together in the dark. A pair of pilots sleep top and tail in another bed and all four are squashed into what is basically a supply cupboard stuffed with two single beds and lamp. Maybe it would be easier on them if one turned and faced away but easy wasn't their way.
Both are mentally and physically exhausted and neither can sleep. Adrenaline pulses weakly in their veins and Luke slips his knee a little higher between Leia's legs. They haven't been on the same planet for months and it makes them wild and careless. After the battle on Endor and the revelations before and after Leia has become far more aware of her heightened senses, especially when it comes to her twin, and it makes her head spin to be lying next to Luke and be so aware of him. Reaching out across the stars he was a dim light and now in front of her he's a lit beacon. Her head pulses and she strokes his arm lightly with her fingers and peers out into the dark towards the other bed, trying to sense if they are awake or not, a threat or not.
They should sleep, should conserve their energy, should stop and move apart and perhaps get separate rooms altogether given their history but they don't. They never do.
Luke's thoughts betray him and wander into places a Jedi's never should like if he could use his powers to keep the other occupants of the room asleep long enough to touch his sister the way a brother never should. At least, not on the worlds they grew up on. For other species, races and groups it wouldn't be frowned on at all. Genetic consequences either don't exist or are ignored and that thought is enough to prod Luke into trying to justify his actions. It is enough to make him reach out across the room but Jedi or not his teachings are limited and his powers still growing.
The consequences of that sort of mistake are too much.
His mind constantly battles what he feels he deserves and what he thinks he shouldn't have. It is just so much harder to resist when Leia is letting out those soft little breathes and scooting down the bed until his knee is pressed against her heat. Luke grows hard and uncomfortable in his cotton trousers, hardly able to contain a gasp of his own when Leia shifts forward again and changes all the angles until her breasts are pressed to Luke's chest and her stomach is rubbing at his hardness.
The consequences are there, right across the room, out in the hall and down the corridor waiting to overhear but Leia grew up avoiding assassination attempts and Luke has never had anything worth risking until now so consequences be damned they fall together anyway.