It took a long explination, but eventually Flash Sentry was on the same page as everyone else.

"So, your really a princess." Flash Sentry said.

"Yeah. I understand if that makes things difficult..." Twilight said.

"Are you kidding? I said that wouldn't matter to me. I meant it. I said I'd follow you anywhere, and I really meant that. Or is this not enough proof of that?" Flash Sentry asked.

"I can't argue with that logic..." Twilight said.

"Yeah...not gonna lie; that was pretty ballsy." Sunset said.

"...I got nothing to add. Just gonna let things sit in my mind for a while..." Shining Armor said.

"So I'm a pegasus now am I? Well, for you Twilight, I'll deal with it. I'm sure my dad will understand. I'm very willing to give up my entire old life just to be with you. I'll find some way to be a high ranking officer of the law somehow. It's still my dream, and I'm willing to make that kind of sacrifice of leaving my old life behind. Because that life does not have you in it, Twilight Sparkle." Flash Sentry said. Rarity fainted. "...Is the pony version of her going to be okay?" The pegasus stallion asked.

"Actually, why did you just faint Rarity?" Sunset asked the white unicorn.

"Because this seems straight out of those Romance Novels that I'm a huge fan of." Rarity noted.

"I...haven't read such a romance novel yet. This seems way beyond like in those." Twilight said.

"I have to say though...anyone willing to leave their entire old life behind for my own sister is alright in my book." Shining Armor said.

"Yeah, that's some fierce loyalty." Rainbow Dash said.

"Some things don't change much..." Flash Sentry noted.

"So, you're really willing to just, give up everything back in that world for me?" Twilight Sparkle asked.

"I meant it too." Flash Sentry said.

"Well, I suppose we can test the waters. I might as well try with someone who sees me for who I am past the crown. Which I'm pretty sure you do." Twilight said.

"Yeah. That would make it hard for just about anyone. But not me. I think me being from that other world and meeting you in that circumstance does give me an advantage." Flash Sentry said.

"He's not wrong..." Sunset said.

"Well then Flash Sentry, allow me to welcome you to Equestria." Twilight said. "More specifically, the Crystal Empire. And there's a lot to go over for the next few weeks." She added.

Author's Note

So it's not really that much of a plot twist. But it is a twist none the less. The idea of Twilight falling in love with the human world's flash sentry came from the story "Intergration" which you can read here: Integration - Fimfiction
That said, no, this does not mean Pony Flash Sentry will show up later. In this alternate universe, Pony Flash Sentry does not exist; there's only one Flash Sentry and that one is now in Equestria over the human world. This was my big twist.