Can he marry a girl whose maternal uncle was breastfed with his oldest brother?

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Check below answers in case you are looking for other related questions:
Doubt as to whether breastfeeding took place
Breast-feeding a child over two years old

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Related Answers:
The blessing of natural breastfeeding
It is permissible for a woman to breastfeed her child before doing ghusl
All the children of the woman who breastfed you, from all of her husbands, are brothers and sisters to you
Doubt as to whether breastfeeding took place
Breast-feeding a child over two years old

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