Each of them breastfed the child of the other; do any rulings result from that?
Question

 
I hope that you can explain the ruling on breastfeeding (radaa’ah) in full, for example, I breastfed my brother-in-law’s son for a month, and my son was breastfed by my brother-in-law’s wife. I have a daughter and a son who are older than the child who was breastfed by my brother-in-law’s wife, and she also had two children before the child of hers whom I breastfed. 
I hope that you can describe the kind of breastfeeding that makes the child a mahram and the rulings that apply to the rest of the siblings? Thank you very much.
Praise be to Allah.Whoever is breastfed by a woman five times, before the age of two years, becomes her child through breastfeeding and she becomes his mother. Her husband (the “owner of the milk”) becomes a father to him through breastfeeding, and everyone who was also breastfed by this woman becomes his brother or sister through breastfeeding, and so on. 
That is because of the report narrated by Muslim (1425) from ‘Aa’ishah who said: “When the Quran was first revealed, the number of breast-feedings that would make a child a relative (mahram) was ten, then this was abrogated and replaced with the number of five which is well-known.” 
And al-Tirmidhi narrated that Umm Salamah said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “The only breastfeeding that creates the relationship of mahram is that which fills the stomach from the breast, before weaning.” Classed as saheeh by al-Albaani in al-Irwa’, 2150. 
What is meant by “from the breast” means at the time of breastfeeding. The Arabs say “Maata fulaan fi’l-thadiy (So and so died at the time of breastfeeding, i.e, in infancy, before weaning).” This is the view of al-Shawkaani. 
Al-Tirmidhi said: This is a saheeh hasan hadeeth, and the view followed by the majority of scholars among the companions of the Prophet (peace and blessings of Allah be upon him) and others is that breastfeeding does not make a child a mahram unless it is done before the child reaches the age of two. Any breastfeeding that occurs after the age of two does not make the child a mahram.” 
Al-Bukhaari (2645) narrated that Ibn ‘Abbaas (may Allah be pleased with him) said: The Prophet (peace and blessings of Allah be upon him) said concerning the daughter of Hamzah: “She is not permissible for me (to marry), because what becomes mahram (forbidden for marriage) through breastfeeding is that which become mahram through blood ties. And she is the daughter of my brother through breastfeeding” 
The definition of breastfeeding: 
Ibn al-Qayyim (may Allah have mercy on him) said: 
One breastfeeding (rad’ah) means when the child takes the breast 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. See also question no. 2864 
Shaykh Ibn Baaz (may Allah have mercy on him) said: 
If a woman breastfeeds a child for five well known breastfeedings or more during the first two years, the infant becomes a child for her and her husband who is the owner of the milk, and all that woman’s children from the husband who is the owner of the milk or from other husbands become siblings of this infant, and the children of the man who is the owner of the milk, whether from the breastfeeding woman or from other wives, become siblings of the infant. Her brothers become maternal aunts for him, and the brothers of the husband, the owner of the milk, become paternal uncles for him, and the father of the woman becomes a grandfather for the child and the father of the husband, the owner of the milk, becomes a grandfather to him and the husband’s mother becomes a grandmother to him, because Allah says concerning female mahrams in Soorat al-Nisa’ (interpretation of the meaning): 
“your foster mothers who gave you suck, your foster milk suckling sisters”
[al-Nisa’ 4:23]
 And the Prophet (peace and blessings of Allah be upon him) said: “What becomes mahram (forbidden for marriage) through breastfeeding is that which become mahram through blood ties.” And he (peace and blessings of Allah be upon him) said: “There is no breastfeeding except during the first two years.” And it was proven in Saheeh Muslim that ‘Aa’ishah (may Allah be pleased with her) said: “When the Qur’an was first revealed, the number of breast-feedings that would make a child a relative (mahram) was ten, then this was abrogated and replaced with the number of five which is well-known. Then the Prophet (peace and blessings of Allah be upon him) passed away when that was the state of affairs.” This version was narrated by al-Tirmidhi but it is also found in Saheeh Muslim. 
From Fataawa Islamiyyah, 3/333 
In conclusion: 
If your son was breastfed five times in the first two years by your brother-in-law’s wife, then he is a brother to all of her sons and daughters. 
Similarly the son of this woman who was breastfed by you, becomes a son to you and a brother to all your sons and daughters, whether they are older or younger than the one who was breastfed, whether they were already present or have not been born yet. 
Your sons – apart from the son who was breastfed by the wife of his paternal uncle – may marry the daughters of their paternal uncle, because they are not mahrams to one another. And your daughters may marry the sons of their paternal uncle except the son who was breastfed by you, because he is a brother to them, as explained above. 
And Allah knows best.