I've used the search function for phrases that, to me, should find results if a question about this story has been asked before. But sorry if this is a duplicate and I just didn't search well enough.
I remember almost all of the story. A girl lives with her parents and big brother near a small town in the mountains of, I think, Colorado. She has a dog. One day a small group of strangers come by their farm. The family is very wary. Events ensue, the strangers turn out to be bad guys. They kill the girl's dog, then I think the dad kills them. The girl has a weird personality and reactions considering her life. She comes across as very shallow and self absorbed in light of strangers trying to take over the farm, kill everyone or maybe just kill the men and rape the women, her parents freaking out, and an adult brother who is clearly clinically depressed and suicidal over something. I wondered at the time if she had a disability that made her so stupid and unable to grasp what is happening around her. Throughout the whole story she is obsessed with going into town to the post office looking for a lost letter. She believes that finding this letter will make things better. The post office is a mess of old letters that never got sent or delivered after America was hit with nuclear missiles and society collapsed.
I do also remember the ending, but I don't want to include it because I don't know how to do spoiler tags
Put your spoilers here!
and I think everyone should read this story. It is truly a great great story. I read it in junior high in the late eighties or early nineties in our textbook for language arts/reading class.