A.aleykum...my question is i have an elder brother whom we have same father bt different mothhis mother died when he was young and my father married another lady who is my mum...but before my father married the first wife she already had a girl..so my brother has another sister...is his sister a mahram to me? since we dont share a mother and father but she shares a mother with my brother?

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The sister of your brother from the father’s side only, if she were his sister from the mother’s side only, would not be considered your sister, and you do not have any relationship with her in this regard. Rather, she is considered a non-Mahram (marriageable) woman to you unless there is another reason that renders her a Mahram (non-marriageable) to you, for instance if she had suckled from your mother and the like.

The fact that her mother is a Mahram to you because she is the wife of your father does not make her [this girl, i.e., the sister of your brother] a Mahram to you.

Allaah Knows best.