These two are DEFINETLY sleeping together. There's no universe, no alternative reality or story arc where their connection wouldn't put Habsburg family to shame. The game itself doesnt even try to ask such question, its as pointless as asking "Is Dawei a faggot?" or "Am i going to win this coinflip?" The answer lies on the surface.
You would go out once in a while, feeling nostalgic for the movie about Big Guy and CIA, and see the glowing lights of Record Play. Sign OPEN hangs on teh door. But there are no clients inside. Why? Because everyone can clearly hear from outside sounds of an old wooden bed furiously creaking horrifically mixingwith sonds of flesh slapping against another flesh. Creak SLAP creak creak SLAP SLAP. All of New Eridu know: if you ring in the bell, you'll have to wait for at least 15 minutes until one of the siblings get down from upstairs, all sweaty, mumbling something about broken AC. It breaks every time for some reason, something with the filter perhaps? 24th time this week. Geez.
Their small business is barely holding together because of their expenses on new beds, they have to replace them every month with a new one. They just break them to pieces.
Sister? Genki tomboy. Brother? Handsome stud. Of course if they're alone the collapse into each other like two neighboring black holes. If you were to separate them, throw one of them to the edge of the world, like Adam and Eve make them wander in endless wastelands, in just a week they will find each other, just to start their mating session on sixth street once again.