How do the class divisions and family quarrels highlight racial tensions in Maycomb?
The class divisions are clearly highlighted in the linked e-notes page http://www.enotes.com/to-kill-a-mockingbird/class-system-maycomb-county.
What is significant in terms of the Depression is the tension caused as society begins to address the issue of racial inequality in a time of social deprivation. It can be seen that Bob Ewell's cruel condemnation of Tom Robinson is more than just a backlash for Tom's insolence in helping Mayella - a white girl - who Bob Ewell, as her father, may treat as lowly as he wishes. Mayella is Bob's daughter, therefore she is his property.He can treat her as he wishes: she is, in effect, his slave. She still remains, however, in Bob's mind, far above Tom Robinson's social network. This hierarchy needs to remain as clearly cut to Bob Ewell as it has always been.
Bob Ewell's scheme is also to rid the town of Maycomb of a hard working man, a black man who formerly would have been shunned by the white people. As the consciousness of social reform dawns slowly on Maycomb; through Atticus and his family, and even through the ministrations of Aunt Alexandra's missionary circle, so even the likes of Bob Ewell can see that there will soon be a swell in numbers of those accepted in society, people who can work and be respected. The growing acceptance of the black population erodes the boundaries which kept the likes of the Ewells, 'poor white trash', from the bottom of the heap.
The economic climate has pushed even some of the most diligent and committed of men over the edges of the poverty line. A simple logic that would fit with Ewell's outlook is that if there are more people accepted into the community, there is less of everything to go around: food, charity, pity and tolerance of those who do not help themselves. Bob Ewell's destruction of Tom Robinson is his bid to reassert the status quo and ensure that the word of the white man supercedes all others.
What Bob may also reassert in his actions is that the pity remains with those who society regards as its lowest common denominator. It is the next layer down from the black men in society: the women, with whom our pity rests. We see and feel the grief of Tom's wife, Helen, and the pitiful wreck that Mayella has become. The family quarrels within Maycomb: Scout growing into the role of a young woman (but fighting being a lady) and Mayella's silent fight against the oppression of her father reflect this other social struggle - that of the rising energy and desire for equality from the women in 1930's America. Mayella challenges the boundaries between black and white men in a bid to make her own choices with her affections. Scout's fascination with and obedience to Calpurnia ensures she learns more of social propriety and acceptance than she does from her starchy Aunt Alexandra.
Both the class divisions and family quarrels in the novel are instrumental in highlighting racial tensions and prejudices. They also serve to highlight Lee's charting of other social change - that of the role and rights of women - in the context of the era of the Depression.