This morning, with some ideas about what I might say about the sentence, I read the book. The book gives the sentence a different meaning than I'd taken as I considered the first sentence on its own. I'd expected the story of an illness. I took "For a long time, my mother wasn't dead yet," to mean that everyone had known the mother was going to die for a long time before she did. My best guess was cancer or some lingering wasting disease.
I have a good imagination. I can roll out long stories from small clues. Sometimes I'm wrong. This was one of those cases.
What the narrator meant could be a spoiler, so I will not tell it here. I will drop the expectation of a long illness and see what's in the first sentence itself.
There are two characters: the speaker and the speaker's mother. The mother is "my mother" – very close to the person who tells the story. There are markers of time: "For a long time" and "yet." "For a long time" means this situation stretched out. "Yet" marks the end of that duration, and it is a short, sharp clip of a word. "Yet" breaks the long stretch decisively.
There is also a problem. "For a long time, my mother was not dead yet," is in the past tense. That past tense implies that, although there was a long period before she died, the mother is dead now.
It's a short sentence, full of short words. The longest is "mother." "Mother" and "dead" are both very strong words, full of associations and emotions. Together, they worry us.
The part of the sentence that most matches the rest of the book is "For a long time." There is a feeling of suspension, of pending grief or loss that continues throughout the story. Although the first sentence misled me a bit, the tone and the worry of it begin creating the mood that will hold throughout the novel.
Graphic elements by Ken Silbert