Good luck with having a girl!! Not being able to see anything on the ultrasound sounds like a good sign to me that it may be a girl. If you have a boy though just think how close he and your son can be!! Have you had any strong cravings? People told me if you crave sweet things it is a girl and if you crave savory it is a boy. All old wives tales I know but the tale has been true in a number of cases I've heard.
Ryan (2) & Jayden (18mths)