His mother breastfed his maternal aunt; what is the ruling on marrying the daughters of his maternal aunt?

Praise be to Allah.

What we understand from your question is that your brother was breastfed by your grandmother and that your younger maternal aunt was breastfed by your mother and your older maternal aunt. 

If what we understand is what was meant, and your brother breastfed from your grandmother for five complete feedings before he reached the age of two years, then your brother has become a brother to your mother and maternal aunts. Based on that, it is not permissible for him to marry the daughter of your maternal aunt, because he has become a maternal uncle to her through breastfeeding. 

If your younger maternal aunt was breastfed by your mother for five complete feedings in the first two years of life, then she has become a sister to you and your brother, and you have become maternal uncles to her daughters. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: If the child was breastfed by his maternal grandmother for five or more complete feedings within the first two years of life, he thus becomes a brother to his maternal uncles and aunts, and a paternal uncle to the children of his maternal uncles, and a maternal uncle to the children of his maternal aunts. So it is not permissible for him to marry any of the daughters of his maternal uncles, because he became a paternal uncle to them through breastfeeding. And it is not permissible for him to marry any of the daughters of his maternal aunts, because he became a maternal uncle to them through breastfeeding. And Allah is the source of strength. 

End quote from Majmoo‘ Fataawa Ibn Baaz, 22/306 

Based on that, it is not permissible for you and your brother to marry the daughters of your maternal aunt, because you have become maternal uncles to them. 

And Allah knows best.