Pointing to a lady, Rani said, “She is the sister of the father of my mother’s son”. Who is the lady to Rani?

Rani’s mother’s son is Rani’s brother, and his father is Rani’s father. The sister of Rani’s father is Rani’s aunt. She is not Rani’s mother, and a cousin would be the aunt’s child, so "Aunt" is the correct relation.