بسم الله الرحمن الرحيم
If your grandmother suckled you while you were a baby then she became your foster mother and your phuphi became your foster sister and her daughter will be your foster Bhanji. Hence your nikah is not lawful with your phupi’s daughter as it is unlawful the nikah of real Mamu and Bhanji in the same way it is also unlawful for foster as the holy Prophet (صلى الله عليه وسلم) said:
يحرم من الرضاع ما يحرم من النسب.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.