When I was younger I saw a movie of relatively grainy quality. I think the movie was centered around a young brother and sister duo, but I don't know what they were trying to accomplish. towards the end though the protagonists meet a female alien in a sort of dark room, I think the room contained a variety of technologies and among those was a TV. The Female alien had big lips and eyes that stuck up over her head like antennae. The alien had sort of a "Marilyn Monroe" essence about her.
I've been trying to find this movie for quite some time now so if you can, please help me out.
There is a female alien in Explorers who is as you describe. However, that movie was about three boys. A girl didn't join them until the end (like, literally the final shot of the movie).
Here is a picture of the female alien from the movie.
(http://www.aleka.org/phoenix/filmo/expl-2g.jpg)
Duddeee!! That's it! Thank you, wow.