The mother and this “step brother” are not related it seems and they
are not mahram to one another , let alone being a mahram to that girl!

If he was a half brother, meaning that he shares a mother or a father
with this woman, he is a mahram to her daughter. Otherwise, he is a
total stranger.

I got a question from a person which confused me so I thought to ask you. A girl whose mother has a step brother (from another father), he is that girl’s step uncle, is he her Mahram or not?

Step Relations and Related Matters | June 28, 2016 4:38 am
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Asalaam Mualakum Sheikh, I currently have my step father’s last name and I will soon in shaa Allah change it to my real father’slast name. I was legally adopted as a child by my step father. I am now 20 years old, do I have to be un-adopted in the courts or is the name change sufficient?
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What role (responsibility) does a stepfather play in his wife’s children’s lives. Specially if conflict arises between the children and stepfather & wife (for some reason) keeps a distance from the husband.