It was 1981-1982 when I read it. My female English teacher told me to read it...not the class. Not sure when it was written? I remember it was written from the sister's perspective. There seemed to be no parents from what I remember. And it was underground or something...not like real earth. I would love to know what it is! The teacher I had died in the late 80's so there's no way to try to contact her. Thought I would try the net. Thanks for your time!!
I'm thinking it might be The Girl Who Owned a City by O.T. Nelson. It was written in 1975 and during the '80s was often an "enrichment" selection on grade school book lists, meaning it was given to children who breezed through the regular class reading lists.
A virus kills all the adults, and society falls apart. A girl and her younger brother become the central unifying figures of a group of children who establish a society in a school building, which they fortify and barricade against marauding child gangs, so that a fair part of the book has the protagonist-children living exclusively indoors in semidarkness. It is written in omniscient third-person, but focuses mostly on the girl's point of view.
For some reason your question made me think of "Podkayne of Mars" by Robert A. Heinlein, although it only fits some of your description. A brother and sister are traveling in a spaceship. The story is told mostly through digital journal entries made by the sister (the brother hacks into her journal and makes some entries also.)
This sounds like "Why Weeps the Brogan?" by Hugh Scott
The brother and sister (the POV character - point) are trapped in a buried museum (thus "underground") with hostile spiders and a mysterious creature called "The Brogan", although you have to work that out (and a lot else besides) by yourself. Some catastrophe has befallen them and their parents are no longer around (point). The book is essentially a big puzzle whereby nothing is initially explained. The story gradually reveals who they are, how they came to be there, who or what the Brogan is and why it weeps. The writing is quite stylised and it won the Whitbread in 1981 according to Blog Post about Why Weeps the Brogan? by Hugh Scott (thus likely to be a recommendation by an English teacher).
I hope it's the right book!
This sounds like one of the books from the Chthon or Phthor series by Piers Anthony.
Ender's Game by Orson Scott Card!