The fact a parent is/was in the military helps a little - the reason is that SAs believe that candidates who have been exposed to military life through their parent(s) are more likely to understand what it's all about and, accordingly, more likely to stay at the academy. In terms of admissions, you get a small boost, more or less equivalent to being an Eagle Scout, team captain, etc. It doesn't matter, BTW, what service your parent was in, whether he/she attended a SA, or whether he/she was officer or enlisted -- it all counts.
There is no minimum GPA, as GPA itself is somewhat meaningless. USNA prefers class rank to GPA, because that demonstates how you are doing in comparison to your peers. A 3.9 isn't as meaningful, if it still puts you in the bottom 25% of your class.
Equally important is your courseload. A student with a 3.9 who is taking AP English, AP Calculus, AP Physics, a 4th year of Chinese and World History will be looked on more favorably by a SA than a student of the same year who has a 3.9 in Music Theory, Art History, English, Algebra II, and first year French. Not saying anything is wrong with the latter courses in general, but they won't help you as much with a SA.
