Olivia and Noah are the children of Liam, who is the brother of Jackson's parent (could be mother or father, it's not specified). This makes Olivia and Noah the grandchildren of Amelia, as their father, Liam, is Amelia's son. In relation to Jackson, Olivia and Noah are his cousins, since their father Liam is Jackson's uncle.

Sophia is married to Liam and is Jackson's aunt by marriage. Jackson might refer to her as Aunt Sophia because she is the wife of his uncle Liam. Although Sophia is not a biological parent to Jackson, she is part of the extended family through her marriage to Liam, and it is common in many cultures to refer to the spouses of aunts and uncles with the same familial terms as their biological counterparts.
Amelia is Jackson's grandmother, and Jackson has an uncle named Liam. Liam is married to Sophia, and they have two kids: Olivia and Noah. Describe how Olivia and Noah would be related to Amelia and Jackson, respectively. Furthermore, why might Jackson refer to Sophia in familial terms, even though she is not his parent? Respond very briefly.