The first scene by Rafael Reis (UE4Arch). I think I read somewhere that he bakes his curtains textures in Corona/Vray
You may achieve similar results in UE4 with a material set to Blend Mode = Translucent, Shading Model = Default Lit and Lighting Mode = Volumetric PerVertex NonDirectional. You’ll need to turn of cast shadows on curtains meshes or they’ll occlude your light during lightmass calculations. Those curtains in Realistic Interior Demo works for directional light only. If there is only skylight coming through the window it won’t work.