be to Allaah.
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.