By default, light will only shine in through a window if the light_environment angle is set so that sunlight shines directly through it. However, real-world windows will often stream in refracted (indirect) sunlight, independent of the suns angle.
Valve commonly uses the following technique to simulate this:
1. Create the opening that you want light to refract through (such as a window).
2. Cover the opening with a brush using the nodraw texture.
3. Texture the inward-facing side of this brush with the lights/white001_nochop texture. This particular texture emits light in-game. You can adjust the strength of this light with the "Texture scale" settings. Values between 0-1 work. Closer to zero, the brighter the light. (Lightmap scale does nothing in this regard.)
4. Tie the brush to a func_brush entity. In its properties, set Render Mode to Don't Render to will prevent the non-transparent brush from blocking the view as a white sheet. (It will still emit its light.) Unless you want the brush to simulate unbreakable glass, also set Solidity to Never Solid.