the book ‘Proper etiquette of a young mare Vol. III‘.

It's generally accepted that book titles are written with each word capitalised, since it's a title. The only time this convention is ignored is when the author wants to be different, but this is rare; since this seems to be a textbook, it would be capitalised.

“Luna, no. It’s late. Shouldn’t you in bed?”

When did Luna become nocturnal? Depending on when she became such, this might or might not be an error.

“Of course. I’d never lie to my sister,”

...But you will fail to notice her depression caused by ponies ignoring her in favour of you, and by ponies not visibly caring as much about the night compared to the day.
If I could warn you two, it'd be not to get into any sibling fights that will result in a thousand-year time-out for one of you and a thousand-year heartbreak and self-loathing for the other.

“Fetura: The Heat Creature. This familiar has been known to help mares with problems involving heat. This is perfect, Luna. This familiar will help keep you warm.”

...They haven't taught you, Celestia, about reproduction yet? Considering this is ancient Equestria, where healthcare and life expectancy are almost certainly nowhere near as advanced as they would be in Twilight Sparkle's time, ponies likely married and mated young to counteract the shorter life expectancy. Therefore, I don't think your parents could have waited all that much longer to teach you about reproduction, about a mare's "heat".

“No! You don’t touch a fillies private area, you monster!

This is referring to a 'private area' owned by one filly, so it's "filly's" - apostrophe s.
"Fillies" refers to multiple young female equines, and when multiple young female equines possess something, it's "fillies'", with an apostrophe at the end to signify this.
So...yeah. Grammatically incorrect on two counts.

I'm kinda disappointed the Fetura didn't manage to grab and breed both sisters, honestly; a rape's always more enjoyable when it involves a pair of sisters, after all.
Still excellent, though, despite that nitpick; definitely will read this again in future.

9968612

Yeah, there was a point where I thought about it, but the original idea was just Filly Celestia herself, since the drawing came first. (Available on Derpibooru) And if I added her in here, it would have been just a shoehorn. However, while this story is marked as complete, you might have noticed the epilogue had left it very open.

Will there be a sequel? Maybe.
Will Luna be involved? Absolutely.

I don't know if I'll do a sequel because I haven't got an ending and I would like it to build from this story. There's a lot going on and just doing the same thing to Luna wouldn't be exciting, it would just be same thing, different character. That's why I'd like to leave it for a bit, think about it and then decide if I wanna go ahead with a bigger story. All I can say is... Watch this space ;) I've got more stories on the way. Each a little different than the last.