Answered by Ustadha Sulma Badrudduja
Question: I have three half brothers (we share the same father) but they are very much like my own brothers. I don’t distinguish them as being any different. At present they are very young but I was wondering whether they are considered my mahrams once they hit puberty?
Answer: I hope this finds you in the best of states.
Half brothers, those that share a parent with you, are your maharim (unmarriageable kin, pl. of mahram). This being the case, you would not need to observe with them the full hijab that you wear in front of non-mahram.
Allah Most High says in the Qur’an,
“Forbidden unto you are your mothers, and your daughters, and your sisters, and your father’s sisters, and your mother’s sisters, and your brother’s daughters and your sister’s daughters, and your foster-mothers, and your foster-sisters, and your mothers-in-law, and your step-daughters who are under your protection (born) of your women unto whom ye have gone in – but if ye have not gone in unto them, then it is no sin for you (to marry their daughters) – and the wives of your sons who (spring) from your own loins. And (it is forbidden unto you) that ye should have two sisters together, except what hath already happened (of that nature) in the past. Lo! Allah is ever Forgiving, Merciful. [4:23]”
This verse is the basis for the rulings on who are one’s mahram. Unmarriageable kin can be divided into three categories: (1) those related by blood [s: half siblings would fall here], (2) those related by marriage, (3) and those related by breastfeeding. [al-Imam al-Kasani, Bada’i` al-Sana’i`]
Please see this answer for further detail.
Checked & Approved by Faraz Rabbani