It's another censored-for-TV hentai with a more pornographic online streaming version of the episodes. It also seems to have been quite successful. It got a second season and additional bits and pieces (voice dramas and ASMRs, whatever they are). I'm reviewing the whole thing, including both seasons.
In summary, it's nice. Season 1's perfectly watchable. Season 2's still okay, but it's scrabbling around a bit for where to go next and you'll probably be fast-forwarding through the sex scenes by this point. I liked the cast, though. They are:
1. MINAMI TAKASAKI, the "Uta no Onee-san" of the show's title. (The English subtitles I saw translate this as "songstress" and I can't really think of anything better, although it doesn't feel right to me. A literal translation would be "singing big sister!") Anyway, Minami is the presenter of a popular TV show. Tiny children everywhere love her. She sings and dances... but her professional responsibilities mean that she's not allowed a private life. Any scandal would kill the show.
2. SHOUJI FUJIMOTO, a widower and single father. He's a really conscientious dad for his tiny daughter Kana (who's not even old enough for school yet), but having a mother figure would be good for Kana too.
This is a hentai series, so you already know the plot. Minami and Shouji will have sex in every episode. The good news, though, is that they're both mature and sensible about it. They look after Kana. They're careful of each other's professional circumstances. Minami's sensitive towards Shouji's late wife (and they visit her grave). Our heroes only go nuts occasionally, e.g. shagging in the changing room while Minami's meant to be clothes shopping. Oi, you two. The shop assistant's right outside. It's not even as if Minami's managing to keep her voice down.
Season 1's arc is just our heroes' romantic relationship. It's nice. Season 2 introduces a gutter press photographer who's looking for a scoop, which is less successful. 1. several episodes in a row have essentially the same cliffhanger sting. Yeah, we got it. They're being watched. We knew that already. The plot could have proceeded faster there. 2. I don't really buy the whole Japanese "sex = career death" thing. Yes, I know it exists... but I don't see why it applies in this situation. Couldn't Minami and Shouji have simply got married? That's clearly where they're heading anyway. Idols don't have that option, but idols are often peddling unhealthy fan fantasies that I hope wouldn't apply to TV shows for pre-schoolers.
Occasionally (e.g. ep.3), I'd wonder who was looking after Kana. Basically, though, it's fine. Our heroes tell each other their secrets and make a good couple. I probably wouldn't watch a third season, admittedly, and the show's running out of steam even in Season 2. It is, though, likeable, healthy and occasionally even clever.