What does Atticus mean when he says, "This is their home, sister. We’ve made it this way for them, they might as well learn to cope with it” (243).

In chapter 22 of To Kill a Mockingbird, as Atticus and the children arrive home after Tom Robinson's trial, Jem is visibly upset. Scout recalls his face being "streaked with angry tears." Alexandra notices that Jem is struggling with the guilty verdict, and questions Atticus about allowing the children to be present during the trial. While her intentions are to protect the children, Atticus says, "This is their home, sister. We've made it this way for them, they might as well learn to cope with it."
Atticus makes this statement because he believes that racism and bigotry are just as commonplace as "missionary teas" in Maycomb County. He wants the children to understand the society they live in. When Jem asks how the jury could possibly find Tom guilty, Atticus explains that it has happened before and it will happen again. He says that when juries make these decisions, it "seems that only children weep."

In chapter 22, Jem bursts into tears after witnessing racial injustice for the first time and is visibly upset with the outcome of the trial. When Atticus and his children walk into the house, Aunt Alexandra notices Jem's tears and asks Atticus if Jem is alright. Alexandra then says that she didn't think it was a good idea for the children to attend the trial in the first place, and Atticus responds by saying,

This is their home, sister...We’ve made it this way for them, they might as well learn to cope with it. (Lee, 216)

Atticus is essentially telling his sister that the community of Maycomb is responsible for Jem's pain because the majority of citizens subscribe to a prejudiced ideology. Jem, who is a child, must learn to cope with the overt racism throughout his community. Atticus's comment to his sister mirrors Dolphus Raymond's conversation with Scout and Dill outside of the courthouse when he tells Scout,

Let him [Dill] get a little older and he won’t get sick and cry. Maybe things’ll strike him as being—not quite right, say, but he won’t cry, not when he gets a few years on him. (Lee, 205)