breastfeeding that affects marriageability refers to breast-feeding a baby for five times or more during the first two years of the baby's life. Accordingly, if your brother was breast-fed by your maternal grandmother as you mentioned in the question; he is considered her son by breastfeeding and a brother by breastfeeding to all her children. It is impermissible for him to marry any of the daughters of his maternal uncle or maternal aunts for they are considered his nieces on account of the breastfeeding ties. Moreover, it is impermissible for him to inherit from his maternal grandmother, since breastfeeding is not a reason that entitles a person being breast-fed by someone to a share in the latter's inheritance. Allah (Exalted be He) says:
Forbidden to you (for marriage) are: your mothers, your daughters
until He (Part No. 21; Page No. 9) says:
...your foster milk suckling sisters
He (Exalted be He) also says:
The mothers shall give suck to their children for two whole years
Besides, it was authentically reported that the Prophet (peace be upon him) said:
"Breastfeeding makes unlawful what parturition makes unlawful."
Moreover, it was authentically reported on the authority of
`Aishah (may Allah be pleased with her) who said: "It was revealed in the Qur'an: 'Ten exact times of breastfeeding affect marriageability.' Then this was abrogated by: 'Five exact times' and the Prophet (peace be upon him) then died while the ruling was settled as such." It may be worth mentioning that one breastfeeding is counted when the baby holds the breast and sucks milk from it. If the baby releases the breast then holds it and sucks milk again, this will be counted as a second time and so on. As for those amongst the brothers of your concerned brother who did not suckle from your maternal grandmother, it is permissible for them to marry any of the daughters of your maternal uncles and maternal aunts, for they are not affected by the case of your brother.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.