I am doing something wrong! Please help - I have gsd pup 16 weeks - very energetic wants to play all the time. We would be happily playing along and then after a while, all of a sudden she would start jumpimg on you growling and snapping at you - and trying grab your leg & start humping! She will only do this to family members. She is very good with other people. Also, she is scared of walking out of the house compound. I now take her in the car to a park but she is too nervous & scared.

This is over excited play.  You need to watch for signs that she is getting overwrought, and calm things down long before with time out.

Also this article may explain things http://www.jersey.net/~mountaindog/berner1/bitestop.htm , she isn't being aggressive this is how pups play.