Overall, the anime of 'A Sister's All You Need' captures the main elements of the light novel. However, it may leave out certain details or nuances to fit the animation format and time constraints.
Well, it's hard to give an exact figure. The number of adapted novels can vary depending on the production decisions and the story arcs chosen. Maybe you could look for official announcements or reviews for a definite count.
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.