It is permissible for a son to marry the daughter of his stepmother from another man than his own father as long as she there is no other reason to prevent the marriage like fosterage.
Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America, states: “it is permissible for a man to marry daughter of his father’s wife, if this daughter was born to her from a previous husband, because there is no kinship between these two young people. This young man is not a mahram (unmarriageable) to that young girl. Allah mentioned in the Qur’an the women who are forbidden for marriage and then He said, “Lawful unto you are all beyond those mentioned, so that you seek them with your wealth in honest wedlock…” (an-Nisa’: 24)”
Sheikh Yusuf Al-Qaradawi, regarding this issue states the following: “This marriage is permissible, and we have witnessed many examples of this marriage. In this case, the man is not marrying his sister as the girl is not his full sister or even half-sister from paternal or maternal sides. The daughter from his stepmother (i.e. born to her from a previous husband) is not linked to the man in question either through blood or fosterage. This marriage is valid and correct as it does not involve any legal reason for the prohibition of marriage.”