In To Kill a Mockingbird by Harper Lee, can Tom Robinson be Calpurnia's long forgotten brother since the reader never learns her last name?
It is unlikely that Tom Robinson is related to Calpurnia.
First, it seems likely that a blood relationship between Tom and Calpurnia would come up in the course of the novel. Calpurnia is involved in Tom's case through Atticus and through her own involvement in Maycomb's black community—if the two were related, it would be unusual not to encounter mention of it somewhere in the story.
Also, Calpurnia is considerably older than Tom, possibly old enough to be Tom's mother. After all, Tom's children are younger than Zeebo's. As Scout narrates in Chapter 12,
Zeebo was Calpurnia's eldest son. If I had ever thought about it, I would have known that Calpurnia was of mature years--Zeebo had half-grown children--but then I had never thought about it.
Another thing about Calpurnia that sets her apart as not a relative of anyone in the Maycomb community is the fact that Cal does not speak as the others do.
There are a few reasons why it is highly unlikely that Tom Robinson is Calpurnia’s long forgotten brother. The first reason is that when Tom Robinson becomes a minor celebrity during the trial, his face becomes well known in Maycomb, Alabama. A large percentage of Maycomb’s white and African American population attends the trial. At no point does anyone mention a resemblance between him and Calpurnia.
Another reason is that Tom Robinson has a home and family in the African American part of Maycomb. It is reasonable to believe that he and Calpurnia live close to one another. If they were long-lost siblings, it is hard to think that they or their families have never crossed paths.
Finally, in the recently published “Go Set a Watchmen,” an adult Scout visits the elderly Calpurnia. At no point does Calpurnia or any member of her family suggest that they are related to Tom Robinson.