Is it permissible for a man to marry his maternal aunt's son's daughter if he was suckled by his maternal aunt's mother and she was suckled by his mother?

The summary of the fatwa is that the amount of breastfeeding that causes prohibition (of marriage) is five or more feedings. This is based on the hadith narrated by Aisha (may Allah be pleased with her): "Among what was revealed in the Quran were ten known breastfeedings that cause prohibition, then they were abrogated by five known ones. So, when the Messenger of Allah (peace and blessings be upon him) passed away, these (five feedings) were among what was recited from the Quran."

If the questioner was breastfed by the girl's mother to this prohibitive extent, then it is not permissible for him to marry her brother's daughter, because she would then be his paternal niece through breastfeeding. The Messenger of Allah (peace and blessings be upon him) said: "What is prohibited by lineage is also prohibited by breastfeeding."

However, if he was breastfed less than this amount, then it is permissible for him to marry her, though it is preferable to avoid it to steer clear of disagreement.

As for the girl being breastfed by the questioner's mother, this has no effect on prohibiting her brother's daughter to him, so it is permissible for him to marry her.