That wasn't what I'd expected. It's an incest anime. A brother and sister will eventually discover that, um. However they get much less story attention than the brother's unfolding romance with his fiancee, while the show seems light and conventional until it suddenly turns on a sixpence late in the day and becomes about this unusual triangle.
Much is explained by the story's origins. It started out as an adult visual novel, i.e. the pornographic computer game equivalent of a gamebook. You play a boy (Jun'ichi Nagase) who's surrounded by girls. The unwanted, hostile fiancee who's going to be living with him is Yuuhi Katagiri, as decided by their parents. The sister is Minato Nagase, who's blood-related in the original PC game but adopted in the family-friendly PS4 re-release. (The anime flirts with the latter possibility, but never commits itself.) Jun'ichi, Yuuhi and Minato will be living together.
However there's also Tsukasa Kiryu (school journalism club), Mitsuki Siina (student council president), Karen Ayanokouji (spoiled super-rich idiot) and Nagomi Shiraishi (possibly an alien). The game lets you shag them all!
The anime, though, is stepping back from that. It's not a harem anime in the romantic sense, with none of the other girls being interested in Jun'ichi. (That becomes slightly less true in ep.10, but not in any way that makes a difference.) There's also no sex at all (so actually no incest is committed) and surprisingly little fanservice. The school uniforms aren't figure-hugging and there's only very occasional nudity, although it does show nipples.
I'm ignoring the very silly OVA. This is easy because nothing in it matters.
The show's pretty serious about the unfolding tsundere romance between Jun'ichi and Yuuhi. They fight all the time, but three-quarters of the episodes are a completely normal light anime romance. Yuuhi-Minato feels more likely than Jun'ichi-Minato. (The sister and the fiancee are very close and supportive of each other.) The sibling subtext becomes visible in ep.4 and other characters are teasing them about it from ep.5 onwards, but neither Jun'ichi nor Minato has any intention of going there and indeed they don't even take it seriously. She's more like his mum.
Instead, the show's about Yuuhi. It really is focused on her relationship with Jun'ichi, at least until ep.10. That's in a light anime way, though, with insane hairstyles, a hot spring episode and cutaway scenes of the Nagase parents being 007 super agents with explosions, gun battles, etc. These scenes will always be non-sequiturs, except in probably reflecting Jun'ichi's internal state in some way. The opening credits also contain a UFO.
Occasionally one senses that something was lost in the compression from a much bigger visual novel. Jun'ichi used to be a delinquent nicknamed the Geno Killer, which which the show does nothing until ep.12 and Jun'ichi's conversation with himself. The supporting cast also don't get enough attention. Journalist Girl gets a focus episode (ep.8), but it's not great because she's been insufficiently developed. The only second-tier character who stands out is Karen Ayanokouji, who's pompous, full of herself and lovely underneath. She's desperate to be nice to everyone and make friends, but she's also an idiot who only understands spending farcical amounts of money. Her self-promotion can be painful. She's quite funny.
All this makes the show's change in ep.10 quite a surprise. Not all viewers were happy with it. On the upside, the story's taking its triangle seriously and doing right by all three characters. I was impressed. They're exploring everyone's psychology pretty well and not skipping anything.
On the "what the hell" side, though, everyone's fine with the brother-sister thing. (Ayanokouji's a bit startled, though.) The level of understanding in ep.11 in particular is, um, beyond what most of us could have managed. Ep.12 then has a love declaration in a public place. With all their friends watching.
That was kind of mental. Admittedly the incest anime genre is a special kind of mental in the first place, but even so this show kept refusing to do what you'd expect of it. It's a normal anime, which of course in English means "deeply abnormal". (Secret agents and a UFO.) It's got the bait-and-switch of two incompatible romances, without cheating in either of them. Then, after you've watched it all, there's that mindless OVA. I think it's quite good.