Atticus, Jack and Alexandra are brothers and sister. Alexandra married Jimmy and the two of them live at the family homestead, Finch's Landing.
Aunt Alexandra and Uncle Jimmy are directly the aunt and uncle of Scout and Jem. Scout and Jem's cousin Henry, Alexandra and Jimmy's son had his own child Francis:
Aunty and Uncle Jimmy produced a son named Henry, who left home as soon as was humanly possible, married, and produced Francis. Henry and his wife deposited Francis at his grandparents' every Christmas, then pursued their own pleasures.
Although Francis is the same age as the children, he is not their first cousin. He is their second cousin. He is Atticus' great nephew.
Jack is Atticus' brother and the favorite uncle of the children. Jack has no children.
Chapter 9 portrays Scout's extended family as they go to Finch's Landing for the Christmas holidays. This is the only time when Scout gets to see her Uncle Jack, who is Atticus's younger brother, and Aunt Alexandra, their sister. Uncle Jack never got married, but Aunt Alexandra married Jimmy and "produced a son named Henry, who left home as soon as was humanly possible, married and produced Francis" (77). So, Henry is the son of Alexandra and Jimmy and Scout's older cousin. Francis, then, is Scout's second cousin.
Francis calls Aunt Alexandra and Jimmy grandpa and grandma. Scout further explains that "Henry and his wife deposited Francis at his grandparents' every Christmas, then pursued their own pleasures" (77). Therefore, Christmas for Scout and Jem means visiting Aunt Alexandra and Uncle Jimmy, Uncle Jack, and their second cousin Francis. To Atticus, Jimmy is his brother-in-law, Henry is his nephew, and Francis is a great nephew.