In episode 12, 7:43 there is a part were Nina Einstein is doing something. Is that what I think it is?
On the Code Geass Wikia it says
Nina is shown masturbating to her picture in episode 12.
"Her" refers to Euphemia. Nina began to develop a crush for her back during the Japanese Liberation Front Hotel Incident when Euphie revealed her identity to the JLF to save Nina from being attacked.
Nina's crush on Euphie is almost obsessive and even what she says sometimes can be seen as innuendo to her wanting an intimate relationship. In one episode, she attempts to meet Euphie but is pinned down by the guards and Euphie once again saves her. As they talk by the fireplace, Nina says she wants "Euphie's goodness" elaborating that she wants Euphie's bodily fluid (whether she means saliva or "something else" is up to the imagination).
Now I haven't read anywhere which actually confirms what Nina is doing, but what the wiki says is widely accepted due to her obsession for Euphie, her body movements during and after the scene (including how Nina adjusts her skirt when speaking to Nunnally) and the sound effects, and there is very little else a girl would do when they have images of the object of their obsession in the dark room like that.