The breastfeeding by which the relationship of mahram is achieved is subject to two conditions:
1 – That there should have been five or more feedings
2 – That it should have taken place within the first two years of life, before weaning.
See the answer to question no. 40226 for more information on these two conditions.
It is not stipulated that the child should have drunk his fill of milk, rather taking in the nipple and sucking milk is regarded as breastfeeding.
If the child takes in the nipple but no milk comes out, that is not breastfeeding, and does not mean anything.
It says in al-Fataawa al-Hindiyyah (1/104): Because when the milk comes out, breastfeeding takes place, otherwise it does not. End quote.
In Fataawa al-Lajnah al-Daa’imah (21/53) it says: If your mother gave the breast to your paternal uncle to keep him quiet, and he did not receive any milk from her breast, then there is no relationship of mahram, and the marriage is permissible. End quote.
If there is any doubt as to the number of breastfeedings, or whether breastfeeding took place, then the relationship of mahram cannot be proven.
The scholars of the Standing Committee were asked:
I have a female cousin (the daughter of my maternal aunt) and I want to marry her, but the problem is that that there is some doubt concerning our relationship through breastfeeding, as breastfeeding was shared between my grandmother (the mother of my mother) and her mother. But this grandmother is not sure about this breastfeeding. We asked her and she said, “I don’t know, I have forgotten. If this girl breastfed from me, there was no milk then.” This is what the grandmother says. I hope that you can find a proper solution for me; is marriage to her permissible or not, based on what the grandmother says?
If the situation is as described, then you may marry the cousin whom you mentioned, because breastfeeding concerning which there is some doubt does not have any effect. Rather the relationship of mahram through breastfeeding is only established if there were five or more breastfeedings, during the first two years of life. If it is not known, then the basic principle is that this marriage is permissible. End quote.
Fataawa al-Lajnah al-Daa’imah, 21/133
See also question no. 804
Based on this, your brother who it is proven breastfed from your maternal aunt is a son to her through breastfeeding, and is a brother to all her children.
But in your case there is no proof that the relationship of mahram was established because there is some doubt and hesitation with regard to breastfeeding.
And Allaah knows best.