Bukhari

Hadith Commentary:
The meaning is that just as marriage with the paternal aunt (phuphi) or maternal aunt (khala) of one's father is not permissible, similarly, it is not permissible to combine in marriage the father's maternal aunt and her nephew's daughter, nor the father's paternal aunt and her nephew's daughter.

Hadith Commentary:
(1)
In the term "paternal aunt" (phuphi), the paternal grandfather’s sister, the maternal grandfather’s sister, their father’s sister—all are included. Similarly, in the term "maternal aunt" (khala), the maternal grandmother’s sister and the maternal grandmother’s mother are all included. The general principle is that it is prohibited to combine in marriage at the same time any two women such that, if one of them were imagined as a man, it would not be permissible for him to marry the other woman. However, it is permissible to marry the daughter of one’s wife’s maternal uncle, paternal uncle, or paternal aunt.

(2)
In the narration of Ibn Hibban, its reason has been mentioned: the Messenger of Allah (sallallahu alayhi wa sallam) prohibited marrying a niece in the presence of her paternal or maternal aunt. He said:
“If you do so, you will be guilty of severing ties of kinship.”
(Sahih Ibn Hibban, as arranged by Ibn Balban: 9/426, no. 4116, and Fath al-Bari: 9/202)
The wife’s father’s maternal aunt is also included in this category.
What is meant here is the foster (milk) maternal aunt, because after this the issue of fosterage (rada‘ah) is mentioned, meaning that "maternal aunt" is general—whether by lineage or by fosterage, whether she is the woman’s own or her father’s. In any case, several subsidiary rulings arise from this.
Wallahu a‘lam (And Allah knows best).