Wear 'sixy Sleepwear'? I mean come on its better than the Obi Wan hoodie she is wearing.

NPCs default to whatever has the better protection. the sleepwear has not protection, and the robed do, so she will default to the robes all the time, every time when not presented with something better.

I hope they let you choose exactly what your companions ware in the next Fallout. That was a feature that really annoyed me.

http://www.newvegasnexus.com/downloads/file.php?id=34912