Is it permissible for my mother to uncover her face in front of the husband of her maternal aunt, who is like a daughter to her through رضاعة (radha'a - fosterage by suckling)?

What is established by lineage is also established by fosterage (رضاع), due to the Prophet Muhammad's (peace be upon him) saying: "What is prohibited by lineage is prohibited by fosterage." Therefore, if your mother's maternal aunt (khalah) was suckled by your mother five full sucklings within the first two years of life, she becomes your mother's daughter through fosterage. Consequently, it is permissible for your maternal aunt's husband to look at your mother in the same way a man is permitted to look at his unmarriageable female relatives (mahārim), such as the face and extremities.