You won't be admitted to USMA for being a legacy. Everyone gets there based on their own merits. Coming from a military family is beneficial up front because you have a better idea of the culture and the commitment you're getting yourself into, but that is easily learned by people coming in from outside the military just by being around the Army for a little bit.
The only "advantage" that I've seen has been for folks who come from military families is that it opens up some alternative nomination sources. If one of your parents retired from the military or if one of your parents received the Medal of Honor, you automatically qualify for a nomination that comes from a pool of essentially unlimited number of nominations. A nomination is by no means a guarantee for admissions It just means that if West Point, after looking at your packet, wants you, they are able to offer you an acceptance packet. No nomination, regardless of how good your file is, no admittance. Additionally, having the different nomination sources allows West Point to charge different nomination sources if there are multiple competitive candidates in a single area, without having to rely on the limited number of seats a Senator or House Representative can sponsor via their nominations in any given year.