Marigolds situates Lizabeth’s personal story of growth and change within the largest context of the extreme poverty of the Great Depression. She includes her mother, father, and brother in the story, but in many ways it focuses on her relationship with an old woman she barely knew, Miss Lottie. As your question asks for a variety of types of importance, the plot need not drive your choice.
The most important paragraph might be the last one, because at the end the reader finally understands the full weight of the experiences Lizabeth recounts, as she can see them clearly from a distance. “The worlds have taken me worlds and years away, [...] And I too have planted marigolds.”
Also, the paragraphs that explore the family dimensions are important to situate the reader in Lizabeth’s immediate world. Paragraph 6 begins, "By the time I was fourteen, my brother Joey and I were the only children left [...].” Another paragraph begins to reveal her parents’ characters, and her insights into them. It begins about halfway through the story:
When I awoke, somewhere in the middle of the night, my mother had returned, and I vaguely listened to the conversation that was audible through the thin walls that separated our rooms. At first I heard no words, only voices. My mother’s voice was like a cool, dark room in summer—peaceful, soothing, quiet. I loved to listen to it; it made things seem all right somehow. But my father’s voice cut through hers, shattering the peace.
It is followed by numerous short paragraphs of dialogue.
The fourth and fifth related paragraphs are hard to distinguish, as they lay out the two parts of the horrible events at the story’s crux: Her verbal assault on Miss Lottie, and her physical assault on her flowers. The first occurs about halfway through the story:
Then I lost my head entirely, mad with the power of inciting such rage, and ran out of the bushes in the storm of pebbles, straight toward Miss Lottie, chanting madly, “Old witch, fell in a ditch, picked up a penny and thought she was rich!”
The fifth, related one comes later in the story, after she overhears her parents’ talk about their poverty:
I had indeed lost my mind, for all the smoldering emotions of that summer swelled in me and burst [...]. And these feelings combined in one great impulse toward destruction.