What is the thesis of "Sister Flowers"? What does Mrs. Flowers represent to Ms. Angelou? What are the "lessons in living" that Mrs. Flowers conveys to Angelou?

The essay about Mrs. Flowers, called Sister Flowers by her peers, is contained in Maya Angelou's autobiography entitled I Know Why The Caged Bird Sings. 
The thesis or purpose of the essay is contained in this quote: "She was one of the few gentlewomen I have ever known, and has remained throughout my life the measure of what a human being can be." Mrs. Flowers had a great impact on Maya Angelou due to her presence, manner, dignity, and bearing. Angelou admired Mrs. Flowers from a distance, and when she bestowed upon Angelou the honor of spending time with her and teaching her life lessons, she became a role model and mentor for Angelou, as well. 
It was Mrs. Flowers who, both figuratively and literally, helped Angelou find her voice. Mrs. Flowers addresses Angelou about her refusal to speak one day while she is walking home from the store. She says to her: 

"Your grandmother says you read a lot, every chance you get. That's good, but not good enough. Words mean more than what is set down on paper. It takes the human voice to infuse them with deeper shades of meaning." 

She loaned Angelou some books and insisted that she read them aloud. She told Angelou she would not accept any excuse for the books being mistreated, which showed her reverence for books and insistence on respectful interactions. 
Some of the life lessons Mrs. Flowers taught that day were to always be intolerant of ignorance but understanding toward illiteracy. She told her to pay attention to what they called "mother wit," a collective of wisdom from generations. She taught Angelou the power in showing a child that they were respected and valued. She taught Marguerite that language was the way people connected with other people, and it was a gift given to humans that were not given to lower animals. 
At the end of their meeting, Marguerite does respond out loud to Mrs. Flowers, which is a result of Mrs. Flowers's high expectations for Marguerite, her respect for the young woman, and her lessons in life that she takes the time to impart to Marguerite.