Well then…

The first chapter, though it would better be called a prologue with the current second chapter counting as the first, made me feel like I'm reading one of my own very first attempts at writing. Your experience of course makes you able to do a much better job with words, but the problems with storytelling are all the same. All stemming from it being a retelling, I think.

First: huge exposition dump. And totally out of place. The easiest thing that can be done to improve this would be not to have the twins fall asleep at the briefing and put all the info about Chaldea and its mission there. Even if the reader would still have to read through the same boring lecture, at least it would be an actual in-universe lecture and if the reader gets bored, at least they feel the same as the characters. Well, I guess technically it is currently also in-universe as it's Mash telling it, but it feels artificially put in this place of the story when there was even a dedicated briefing where all of this was already told if only the twins didn't fall asleep at the start (you can still make them fall asleep after you've already conveyed all the important info, so that they can still then be kicked out and all). Another thing this would accomplish would be separating the pieces of exposition so that all the stuff related to Chaldea can go in the briefing and Mash can then only tell about her being a demi-servant. Of course ideally the size of the exposition dump should also be reduced as much as possible. Are all the details about how chaldeas works and that its not enough by itself but also needs something called sheba? I bet even the original authors have no idea how all of this works, they've just put a bunch of mysterious words together, so why should we care. The important thing is only that it shows the incineration of humanity and the characters now have to stop that.

Second: rushing through things, some of which call for more detailed description, others should be cut completely. Which I think stems from the issue I wanted to list as third, but now that I think of it, they're mostly the same, and that is: you're not writing a story, but retelling a game/anime. There is no point in the whole "they fought some enemies and gained some exp so they then fought stronger enemies and defeated them too and finally they fought even stronger enemies but by that time they were leveled up enough to defeat those too… oh, and among the enemies there were normal skeletons, shadow servants and some weird skeletons called dragon tooth warriors" part. Save it for the time someone asks you how the game works. Why is it important to list the types of enemies? You're not describing the battles to show how each provided a new or different challenge and how the characters overcame it or something. So just say "they fought their way through to the mountain and discovered a cave". Or focus on the impact the fighting had on the characters, like "they had to fight their way to the mountain and Olga Marie was constantly complaining about it, but Mash said that there's a positive side to it as well, as through practice her and Carmilla were getting better at fighting together, preparing for the fight with the servant waiting for them in the mountain, to which Carmilla added that another positive side is that killing everything in their way is fun, just too bad skeletons don't bleed". Listing the types of enemies on the other end only leaves the reader with questions: why are there skeletons? there's now a new weird type of skeleton, so you mean normal skeletons are normal? The point of the prologue is to introduce the reader to the setting if they haven't played the game, but instead it only makes them more confused. You're also introducing new things that you don't care to explain either, mostly about the twins. They live in the same room why? Were they provided with two rooms, but decided to use only one so they can be together? Did they ask for a single room, provided with one with two beds, but only use one bed? Did they explicitly ask for a room with a double-sized bed, never trying to hide their relationship? Also how does it all work with them both being masters of the same servants? Is it some special power of being twins, because they like have one soul for the two of them or something?

Fourth: all the "oh, by the way, there was that too" moments stemming from you trying to rush through things but then facing the problem that the thing you skipped happened to be important later. Like "oh, by the way Olga Marie was with them all that time", yeah, you did say she joined them, but then never mentioned her again until absolutely necessary, by that point even I forgot she was there, even though I did play the game. Lev Lainur appeared out of nowhere. If you haven't introduced him earlier, at least bother to more properly explain who he is the moment he appears.

All in all if you want it to properly work as the introduction into the setting for people unfamiliar with it, it should be more thorough and detailed. Either that or give up on that and repurposed it for people who are already familiar with the setting to introduce them to what is different in your story, so focus on the twins and Carmilla, and in places of explanation dumps put something like "the briefing explained about Chaldea and its mission to protect humanity" just to inform the reader that the characters now know that, and what exactly are the details the reader already know, and actually even for those who don't know, is it really all that important?

Now, for the second chapter, it's much better from the storytelling standpoint, because now you're actually telling a story and not retelling a game or anime. Just too much alcohol for my taste, but I'll have to hope that it was just because of Jink Ke and will never happen ever again. Also the part with Mash still felt too rushed. It's fine if you want her seduced so easily, but at least give her some dialogue too.

Oh, and speaking of Mash. Plural of senpai in English is senpais, please use plural when Mash is addressing both twins.

Conclusion: the main characters are incestuous twins, so 10/10.