It's mainly about the story centered around the characters in relation to the concept of a sister. Maybe it involves the relationship, the experiences they share, and how that relationship impacts their lives.
The translation of 'A Sister's All You Need' is quite good. It manages to capture the tone and style of the original work, although some cultural references might be a bit tricky to translate perfectly.
I'm not sure how popular it is. It might be popular among certain groups who are into light novels with a focus on family - like relationships, especially those centered around siblings.
Well, it depends on your taste. If you like light novels with a focus on character dynamics, especially those involving the concept of a 'little sister' figure, then it can be quite enjoyable. The writing style is engaging and it offers a different take on the typical light novel themes. There are also some great character developments throughout the story that make it a worthy read.
I'm not sure specifically as I haven't read it in detail, but it's likely that there are characters who are sisters, and perhaps other family members or friends who interact with them.