Your question asks, "What did Aunt Alexandra ask Atticus to point out to the children in Chapter 12?" In Chapter 12, Calpurnia takes the children to First Purchase African M. E. Church and Aunt Alexandra is not mentioned. I think you meant to ask "What did Aunt Alexandra ask Atticus to point out to the children in Chapter 13?"
In Chapter 13, Aunt Alexandra asks Atticus to teach his children about their family history. Atticus approaches Jem and Scout before bedtime and says, "No, I just want to explain to you that---your Aunt Alexandra asked me...son, you know you're a Finch, don't you?" (Lee 177) Atticus attempts to explain the importance of being a "Finch" and teach his children some of their proud heritage. Scout mentioned that Aunt Alexandra was obsessed with heredity and holds the Finch family name in high esteem. She criticizes Atticus for his child-rearing techniques and wants him to teach Jem and Scout manners. Alexandra was offended when Jem elaborated on the made up story Atticus told him about Cousin Joshua. Atticus tries to explain that the Finches were not a "run-of-the-mill" family and were a result of several generations of "gentle breeding." The children notice that Atticus is acting differently, and Scout begins to cry. Atticus gives up on teaching his children about their family history because he thinks it is foolish. He tells his children to forget everything he said about their family history and jokes that he gets more like Cousin Joshua each day.