Reviewing one-shot feels weird, and the fact that it's hentai makes it even funnier, but this manga was memorable enough for me to want to write something about it.
The story is incest focused hentai with very simple setting. There's not much attention given to exposition. We get just enough information to know who is where and why.
Story includes some comedic parts. They are generally nice addition. Comedic interactions between Chika and nameless "Onii-chan" give this manga a nice and warm feeling.
Sex scene itself is vanilla - no rape, tentacles or any other weird stuff. The only notable elements are tan lines and small breasts.
in Natsu no Imouto is really good. While not perfect, as it lacks certain details, it makes a good job of showing appealing parts of characters. Author kept good balance between cute and erotic traits, which is very important in incest stories.
"It depends on what turns you on" would be the right thing to write about enjoyment. While comedic parts are nice, it's just a hentai. Short duration and small amount of characters doesn't leave much space for catering to different tastes.
If you like incest or are looking for well drawn tan lines, I believe you will like Natsu no Imouto.