If a family adopts a child whilst their own biological children are present, then are they mahram or not?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
The adopted child will not be considered a mahram for his/her siblings through adoption, seeing as there is no blood relationship between them. This is with the exception of if the adopted child was breastfed by the mother before the age of 2, then he/she will be considered a mahram.
Only Allah knows best
Written by Maulana Imran Mughal
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Shaykh Nidham, Al-Fatawa Al-Hindiyyah (Beirut: DKI, 2010), 1:301-311.