This review contains heavy spoilers for volume 1, and some of the other stuff in the second half. I tried my best to avoid it, but I can't talk about this story without basically going over the entire plot first.

The first impression I got of this book was from the synopsis and mean score - it doesn't actually say anything about what the story will be about, or even what to expect when reading the story, except that it's probably the kind of ecchi incest comedy that people tend to rate low. Which it is.

The story (mostly) takes place in 2202, where moe has spread and the people are split between the childhood friend, little sister, and big sister factions. There is a single book that initially popularized these ideas (I want to have onii-chan's baby, or Oniaka for short), written long in the past (as in like 2050). The protag has a crush on the protag of this story.

To start off, here's the characters:
Gin is the protagonist. This is a harem, so naturally he has the usual harem protag properties of being too nice to everyone and being too dense to realize people love him. But he's pretty bearable and not cringe, which was enough to make me want to continue reading rather than stopping from the ridiculous story. He learns how to be accepting of those with a radically different view than he does near the start of the story, and I love that concept.
Kuroha is the titular little sister. If it wasn't obvious, she can read kanji, unlike the protagonist. She's has just about the right amount of tsun and she acts like most romcom characters do around ecchi situations (the rest of the characters actually don't), and gets bonus points for never attacking Gin for something that isn't his fault. She hopes to work as a translator (in addition to kanji not being in use, some language structure changed). She dislikes 23rd century culture, but cares too much about Gin to really do anything about it.
Yuzu is a character from modern times (as in 201X). She's the character that the protag of Oniaka was designed after, and met the main cast after they time travelled to the past (and then went to the future with them, since she loves Gin (and Gin loves her too)). She has a dead brother who was a super masochist and she learned how to be a sadist through that, but she's just an airhead.
Meguri is a genius scientist obsessed with the 2D to the point that she does her science aiming to go to a 2D world herself. She invented stuff like the time travel method they use, and in general just sorta does whatever. But she's also gone to the future, and knows what stuff is like there... and wants to change it.
Amaneko is the protag's blood-related sister (Gin's adopted btw), born from the events the cast do in the past in volume 2. Unlike the other main girls, she pushes way too hard for me to consider liking her. She wants to revolutionize language to be like it is in the future.
Miru is Kuroha's elementary-school aged sister. She's the resident kuudere ~~which makes her best girl~~, and often teases Kuroha.
Odaira is an old but popular lolicon author who gains the ability to transform into a little girl (which he of course keeps on almost permanently) as a side effect of the time travel. He likes Miru, obviously, though that's not reciprocated. He joins the crew and does pretty much nothing except be there with funny lines¹.

My biggest gripes with this story are its actual world system. Considering it includes time travel and it doesn't take itself seriously, I can handwave all the details away... but there's a plot point near the start that just makes no sense. Oniaka is actually very heavily inspired by a novel that the main cast (as in Kuroha and Yuzu) wrote while they were in 201X, but that novel would never have been written without Oniaka already existing as the inspiration for the world to have changed as it did. In fact, at the end of volume 1 the future actually changes due to that book they wrote having been removed so that the author of Oniaka never sees it and thus can't use it as inspiration. This basically means that it doesn't make sense for the world to end up how it actually does, and there's no explanation given for this glaring issue.

Aside from that problem, the rest of the setting seems fairly reasonable though it's extremely centered around time travel without good explanation on how the world works. Doing stuff to avoid changing the future is explored in more common shows about time travel, and it being entirely centered on the protaganist's actions is just par for the course.

After volume 2, the cast returns to modern times and the story suddenly actually becomes like a harem novel instead of the ridiculous stuff that happened previously. But there still is a story; it expands on the earlier events of learning to accept other people with drastically different tastes and letting those people stay together in the same country (since for some reason all these people want to change the entire country). The harem is pretty typical, but is spiced up with time travellers from the future trying to influence history and the like.
Gin's extremely illiterate, so his harem works to (interpret and) translate his books into something readable. Through translation something can actually avoid being lost through time, and even as the times change some people might enjoy the old etc etc. Of course, it's his diary that gets translated to modern Japanese, and then sent to the past to win a Newcomer's prize at a competition².

Also, there's these random portions that just go over a radio show and they seem pointless except to give Odaira and Miru some more screentime, and fill up some pages. I'm not too sure why they felt like doing that for almost every chapter, but it's never relevant in the slightest... though it made me laugh sometimes.

The english translation itself feels fairly iffy near the beginning, and I found it pretty challenging to actually read. It contains the occasional character that isn't actually translated - this makes it fairly difficult to interpret since I don't really know Japanese in the slightest. And there were a few points where I had to go back and remember what something meant.
But overall, it's pretty good. The later volumes mostly contain challenges by design (stuff that was left in the script of the future in the japanese version too) and impossible-to-translate puns, which are fine and par for the course. I think it was really just the translator trying to get their bearings or something of the sort. Either way, it's a good translation and I'm glad they didn't try to localize literally everything like I see some publishers do (it's horrible when people localize honorifics).

#Final thoughts
I would recommend this story to anyone who likes surreal comedy and imouto harems (like oreimo or imosae). It's still not particularly good, but I like it when writing is a theme. Apart from the usual issues with time travel, it ends up just being a combination of those two themes to make for an engaging experience that leaves you wondering what the heck you just read after you finish.

#Footnotes
¹ I find that the funny lines are much the same from any lolicon character, but consider /manga/104197 if you like them.
² This story did win one such newcomer's prize, and thus it was the author's debut work. I'm a bit surprised, but this story was better than a lot of other things I suppose.