Although I was not a falcon myself, I had the opportunity to attend NWP last fall with a class primarily made up of falcons. Although there are no "official" stats out there (as far as I know at least), you have a high chance of getting in; it really is what you make it out to be though. The Durbecks at NWP told us many stories of falcons not getting in simply because they took the golden ticket as a guaranteed slot into the academy, so they decided to become lackadaisical, slack on their studies, and miss deadlines. You've been given an incredible opportunity to attend a prep school for free! Take it and give it everything you've got. Some of us had to pay full tuition to receive the same education and it is disappointing to see people who've been given this amazing opportunity throw it away. Some of my falcon classmates didn't turn in assignments, didn't study for exams, and just didn't give their full effort, and I honestly wouldn't be surprised if they didn't get in this year. A falcon foundation scholarship is what you make it; give it your all and you have a very high chance of getting in. Hard work pays off!
I don't know a single prepster who isn't grateful that they were able to go to a prep school and spend another year working on themself. Doolie year is a huge learning curve, but the experiences at a prep school make it that much more tolerable. It is such a great opportunity that will help you to thrive at the academy. Take it and run with it.
As far as what you should tell your family, I would take the time and explain prep school in-depth and what it is. I know it can get pretty repetitive and annoying, but you're going to be gone for a few months with pretty limited contact. It'll really help calm their nerves if they truly know what you're up to. Also, it really is a great thing! USAFA likes you enough to drop a few thousand dollars into your education and training. Prep school is basically the "Intro to Service Academies 101" course that you're getting to take for free because they think you have some potential to succeed at the academy.
Now the choice of what prep school you go to is ultimately up to you, but I would highly recommend NWP (I'm slightly partial haha). It is an exceptional institution and you will learn a lot, both academically and about academy life! It's on the top of a mountain in CA and I'm told it has a beautiful view (I wouldn't know because I did NWP online due to covid). The academic will be incredibly challenging and you will be working all day every day, but it will be worth it. You'll get to hear from current and past cadets from all over the country on how they succeeded in the academies. I'm sure that the Durbecks would be happy to give you more information and I'd also be happy to give some as well. Definitely do your own research though and figure out what fits best for you. Best of luck to you in your future endeavors!