Son marrying his father’s stepdaughter
The daughter of your father’s wife is known as your father’s stepdaughter, and she is a mahram for your father only, if he has consummated the marriage with her mother, whether he brought her up himself or she is an adult and he did not bring her up. This is the view of the majority of the earlier and later scholars, and is the view of the four imams. When mentioning the women who are mahrams for men, Allaah says (interpretation of the meaning):
“Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your foster mothers who gave you suck, your foster milk suckling sisters, your wives’ mothers, your stepdaughters under your guardianship, born of your wives to whom you have gone in — but there is no sin on you if you have not gone in them (to marry their daughters), — the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allaah is Oft-Forgiving, Most Merciful”
But with regard to you, this stepdaughter is not a mahram for the son of her mother’s husband, so it is permissible for you to marry her, and there is no problem in that.
The Standing Committee was asked about marrying the daughter of the father’s wife. They replied:
It is permissible for the son of the man mentioned to marry the daughter of the woman mentioned, even if his father is married to her mother. Allaah says (interpretation of the meaning):
“All others are lawful”
The girl mentioned is not one of the mahrams listed in this verse, or in the Sunnah.
Al-Fataawa al-Jaami’ah li’l-Mar’ah al-Muslimah, 2/600
And Allaah knows best.