That scene was part of a Chinese mod, but cannot be seen in-game. For some reason at some point those assets made it in but are unused. Use
this mod if you use the latest 3.0 patch, or the previous version linked there (LegionMP's mod) if you use the 3.0 version linked in the opening post (which is somewhat outdated). That mod adds the scene, and all you do is have Sylvie wear the sleeveless dress without underwear when going to bed.
It also adds scenes for a festival and other scenes from that Chinese mod, but you must start a new game due to adding new variables.
