He is married to his maternal aunt through breastfeeding
Question

                Until recently there were concubines in our country, and this issue has come up; I would like to know the shar’i ruling on it. 

A man had a wife whose name was Aaminah, and he had a concubine whose name was Sa’diyyah. 

Aaminah (his wife) breastfed a girl called Zaynab, and the slave woman Sa’diyyah had a daughter whose name was Haleemah. 

The daughter, Haleemah, got married and had a son called ‘Ali, who grew up and married Zaynab, who had been breastfed by Aaminah. 

They have been married for many years, and have several sons and daughters. 
My question is, what is the ruling on this marriage? 

Is it permissible for me not to tell them, if this marriage is not legitimate?.
Praise be to Allah.One of the rulings on radaa’ah 
    (breastfeeding) is that the milk of the wet nurse has an effect on the child she breastfeeds and on his or her children. If Zayd was breastfed by 
    Faatimah, then Zayd and his children are regarded as children of the wet nurse (Faatimah), but his brothers are not connected to them through 
    breastfeeding. 
But for the wet nurse, the bond established 
    by breastfeeding includes all her relatives, so whoever is related to the wet nurse is also related to the child whom she breastfed. So the 
    husband of the wet nurse is regarded as a father of the child through breastfeeding; her brothers are regarded as maternal uncles through 
    breastfeeding, and her father and grandfathers are regarded as grandfathers of the child through breastfeeding. If her husband has another wife, 
    then this woman is regarded as the wife of the child’s father through breastfeeding, and her children are regarded as his brothers on the father’s 
    side through breastfeeding… and so on. ‘Urwah ibn al-Zubayr narrated that ‘Aa’ishah said: Aflah the brother of Abu’l-Qu’ays asked permission to 
    enter upon me after the ruling on hijab had been revealed, and I said, “I will not let you in until I ask the permission of the Prophet  
    (peace and blessings of Allaah be upon him), for it was not the brother of Abu’l-Qu’ays who nursed me but the wife of Abu’l-Qu’ays.” The Prophet 
     (peace and blessings of Allaah be upon him) entered upon me and I said to him, “O Messenger of Allaah, Aflah the brother of Abu’l-Qu’ays 
    asked permission to enter upon me, but I refused to let him in until I asked your permission.” 
The Prophet  (peace and blessings of 
    Allaah be upon him) said: “What stopped you from letting him in? He is your (paternal) uncle.” 
I said, “O Messenger of Allaah, the man is 
    not the one who nursed me, rather it was the wife of Abu’l-Qu’ays who nursed me.” 
He said, “Let him in, for he is your uncle, 
    may your right hand be rubbed with dust (i.e., may you prosper)!” 
‘Urwah said: For that reason  ‘Aa’ishah used 
    to say, “Consider those things which are illegal (for marriage) because of blood relations as illegal because of the corresponding relations 
    through breastfeeding.” 
Narrated by al-Bukhaari, 4796; Muslim 1445. 
This hadeeth confirms that there are parents 
    through breastfeeding, and that the husband of the wet nurse is regarded as a father of the child through breastfeeding. The wife of Abu’l-Qu’ays 
    breastfed ‘Aa’ishah, so she became a mother to her, and her husband became a father to her through breastfeeding, and his brothers became paternal 
    uncles to ‘Aa’ishah through breastfeeding. Hence the brother of Abu’l-Qu’ays said to ‘Aa’ishah, “Do you observe hijab before me when I am your 
    paternal uncle?” Narrated by al-Bukhaari, 3644. The Prophet  (peace and blessings of Allaah be upon him) confirmed that he was a paternal 
    uncle to ‘Aa’ishah when he said, “Let him in, for he is your uncle, may your right hand be rubbed with dust (i.e., may you prosper)!” 
See al-Mughni ma’a al-Sharh al-Kabeer, 
    9/199; al-Mawsoo’ah al-Fiqhiyyah, 22/248. 
With regard to the matter mentioned in the question, the answer is as follows: 
The daughter, Zaynab, who was nursed by the 
    wife, Aaminah, is regarded as a daughter of Aaminah because of this breastfeeding. She is also a daughter to Aaminah’s husband through 
    breastfeeding. So the husband is her father through breastfeeding, if the child was nursed five times or more when the infant was under the age of 
    two years. Based on that, the husband’s daughter Haleemah from his concubine Sa’diyyah is the sister of Zaynab through her father through 
    breastfeeding, and Haleemah’s children – including ‘Ali – are mahrams to Zaynab because she is their maternal aunt through breastfeeding. So it is 
    not permissible for any of them to marry her. 
You mention in your question that ‘Ali married Zaynab. This marriage is invalid and he must 
    leave her at once, because he is regarded as having married his maternal aunt through breastfeeding. 
With regard to the children who were born as 
    a result of this marriage, they are legitimate children who should be named after their father ‘Ali, because this is regarded as an ambiguous 
    marriage and children born as a result of that are to be named after the father, as the scholars said. 
Shaykh ‘Abd al-‘Azeez ibn Baaz was asked: I was breastfed by a woman whose husband took a 
    second wife and had children from her. Are they my brothers? 
He replied: If you were breastfed five times 
    or more, and the milk belonged to the husband because it was produced as the result of a baby being born to him, then they are your brothers 
    through your father through breastfeeding.” Fataawa Islamiyyah, 3/323. 
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah 
    have mercy on him) was asked a similar question: 
After I had consummated the marriage with my 
    wife, I found out that she is my sister through breastfeeding, because I was breastfed along with her sister. Is she my mahram in this case? 
He replied: Yes, if the matter is as you say, 
    and you were breastfed along with the wife’s sister from her mother, in the sense that you were breastfed by the wife’s mother or her father’s 
    wife, then you are her brother and the marriage is invalid. But you should realize that breastfeeding does not count unless the child is breastfed 
    five or more times in the two years before weaning. If it was less than that, then it has no effect and it does not establish the relationship of 
    mahram. 
If you are certain that you were breastfed 
    five times or more by the mother of the woman whom you have married in the first two years of life, then you must leave her because this marriage 
    is not valid. Any children who were born before you found out are to be named after you according to sharee’ah, because these children were 
    born as the result of an ambiguous marriage and children born as a result of that are to be named after the father, as the scholars said. 
Fataawa Islamiyyah, 
    3/329. 
You have to tell them about that, because 
    they have the right to receive sincere advice from you and so as to denounce an evil action, because their staying in this invalid marriage is an 
    evil action. The Prophet  (peace and blessings of Allaah be upon him) said, “Whoever among you sees an evil action, let him change it with his 
    hand (by taking action), and if he cannot then with his tongue (by speaking out) …” Narrated by Muslim. 
May Allaah help us all to do that which He 
    loves and which pleases Him.