Most of his maternal cousins were breastfed by his mother and he was breastfed by his maternal aunt; what is the ruling on his brother?

If a person is breastfed by a woman for five definite feedings, he becomes her son through breastfeeding and a brother to all her children, both males and females. 

The evidence for that is the reports narrated by Muslim (1452) from ‘Aa’ishah (may Allah be pleased with her) who said: Among the things that were revealed of the Qur’aan was that ten definite breastfeedings make a person a mahram, then that was abrogated and replaced with five definite breastfeedings.  

The definition of breastfeeding is as Ibn al-Qayyim (may Allah have mercy on him) said in Zaad al-Ma’aad (5/575):

One breastfeeding (rad’ah) means when the child takes the beast and starts suckling, and then lets it go without being made to do so. That is one breastfeeding, because the Lawgiver referred to one breastfeeding in general terms, so it is to be interpreted according to custom, and this is the custom. If the child stops briefly in order to breathe or to rest or because of some distraction, then quickly goes back to the breast, this is regarded as one breastfeeding, just as when a person who is eating pauses briefly then quickly goes back to eating, this is not regarded as two meals, rather it is one. This is the view of al-Shaafa’i.  And if the infant moves from one breast to the other, this is one breastfeeding. End quote. 

Based on that, if you were breastfed by your maternal aunt five times, then you are the brother of all her sons and daughters, and it is not permissible to you to marry one of them. 

The ruling on breastfeeding has to do with the person who breastfed and his children. As for his siblings, the ruling on breastfeeding does not include them. 

Based on that, if the daughter of your maternal aunt whom your brother wants to marry breastfed from your mother, then she is his sister through breastfeeding and it is not permissible for him to marry her. But if she was not breastfed from your mother, then there is nothing wrong with them getting married, because there is no impediment to their doing so and that is not affected by the fact that you were breastfed by her mother or that her siblings were breastfed by your mother. 

And Allah knows best.