Assalamualaikum ustaz. What is the status of a mil aunt? Is she considered a mahram? The reason is adopted a son who is still a baby but I couldn’t breastfeed him. Hence, I gave my sister to nurse him. Does it make me his mahram due to his relationship with my sister who is now his milk mother, hence I’m his milk aunt? Hope for an explanation.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Nursing (breastfeeding) in Arabic is known as al-Rada’ (الرّضَاعْ) according to al-Mausu’ah al-Fiqhiyyah al-Kuwaitiyyah is the act of a woman feeding a baby her milk (into the baby’s stomach) with certain conditions. 
Whilst according to the book al-Fiqh al-Manhaji, al-Rada’ means a woman feeding a baby her milk (into the baby’s stomach). 
Nursing that fulfils its conditions will result in a familial relationship between the woman and baby (where they become mahram). This is as stated by Allah SWT:
وَأُمَّهَاتُكُمُ اللَّاتِي أَرْضَعْنَكُمْ وَأَخَوَاتُكُم مِّنَ الرَّضَاعَةِ
“…your [milk] mothers who nursed you, your sisters through nursing,” 
Thus, the effect from this nursing causes the milk baby prohibited to be married the same as a child she gives birth to. This is as stated in a hadith narrated from Ibnu Abbas R.Anhuma, where the Prophet PBUH said to the daughter of Saidina Hamzah RA:
لاَ تَحِلُّ لِي، يَحْرُمُ مِنَ الرَّضَاعِ مَا يَحْرُمُ مِنَ النَّسَبِ، هِيَ بِنْتُ أَخِي مِنَ الرَّضَاعَةِ
“She is unlawful to me for she is the daughter of my brother in suckling; and what is unlawful by reason of blood relationship is unlawful by reason of suckling relationship.” 
Imam al-Nawawi stated in his commentary, “Nursing is one of the reasons of the prohibition for a milk son to marry his milk mother forever (muabbad mahram). Thus, it is permissible for him to see his milk mother, live in the same house and travel with her the same as his own mother.” 
The effect of the nursing, the relationship of a milk child with his milk mother and husband as well as her lineage is subject to basic rulings, which are:
It is prohibited to marry them (milk mother and father as well as their lineage)
Syeikh Muhammad al-Zuhaili stated that it is permissible for a milk child in terms of the permissibility to see (what is prohibited from an ajnabi) to his milk mother and her lineage and it is permissible to live with her. 
The author of al-Fiqh al-Manhaji stated: If a woman has nursed a baby who isn’t hers, the baby becomes her milk child and her husband the milk father of the child. The effect of this nursing is a familial relationship as the following:
A milk child is prohibited from marrying his milk mother and lineage, the same as his own mother. The milk mother lineage that is prohibited for him to marry is as the following:
- The female siblings of the milk mother for they are his milk aunt on the maternal side.
- The daughters if the milk mother for they are his milk sisters.
- The granddaughters of the milk mother, for they are the daughters of his milk siblings (milk nieces).
- Female siblings of the milk father for they are his paternal milk aunt.
- Daughters of the milk father even if they are from another mother, for they are his milk sisters.
- Granddaughters of the milk father for they are his paternal milk nieces.
- The mother of the milk father for she is the mil grandmother.
Milk mother and her lineage are prohibited from marrying the milk son and his lineage. The reason is a milk mother is likened to the child’s own mother, surely all of their lineage is also prohibited to be married.
Just like a milk son is prohibited against marrying the daughter of the milk mother for she is his milk sister. The same applies to the milk son from the paternal daughter of the milk mother because she is the paternal milk aunt and the same continues on.
The milk mother and her lineage is permissible to marry the family members of the milk child, such as his brother, father, and paternal uncle for they are strangers towards the milk mother and her lineage. 
According to the above question, the relationship of the adopted son with you is that you’re now his maternal milk mother according to the rulings set by syara’. This relationship is described as الخَالَة مِن الرّضَاع, which means a woman who was nursed together with her own mother or milk mother or nursed by one of the grandmothers. 
This is based on a hadith of Rasullullah PBUH narrated by Saidatina Aisyah R.Anha:
اسْتَأْذَنَ عَلَيَّ أَفْلَحُ فَلَمْ آذَنْ لَهُ فَقَالَ أَتَحْتَجِبِينَ مِنِّي وَأَنَا عَمُّكِ، فَقُلْتُ وَكَيْفَ ذَلِكَ؟ قَالَ أَرْضَعَتْكِ امْرَأَةُ أَخِي بِلَبَنِ أَخِي، فَقَالَتْ سَأَلْتُ عَنْ ذَلِكَ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ فَقَالَ صَدَقَ أَفْلَحُ ائْذَنِي لَهُ
“Aflah asked the permission to visit me but I did not allow him. He said, “Do you veil yourself before me although I am your uncle?” `Aisha said, “How is that?” Aflah replied, “You were suckled by my brother’s wife with my brother’s milk.” I asked Allah’s Messenger (ﷺ) about it, and he said, “Aflah is right, so permit him to visit you.” 
Thus, the ruling between your adopted and yourself is that you’re his milk aunt and he is your permanent mahram and the rulings of a mahram are applicable such as aurah, the permissibility to be alone together (khalwah) and physical contact. May Allah SWT grant us all understanding in his religion. Amin.