Ruling on marrying the grandfather’s stepdaughter
Question

My grandfather has two wives, ‘Aa’ishah and Maryam. My mother is the daughter of ‘Aa’ishah. I have three maternal uncles from the second wife (Maryam). When my grandfather died, Maryam married another man and bore him a daughter. Is this daughter my maternal aunt? I hope you can answer this question for me, because I intend to marry her.
Praise be to Allah.The girl you asked about is not a maternal aunt to you, because she is not the daughter of your grandfather or your grandmother; rather she is the stepdaughter of your grandfather (that is, the daughter of his wife). The stepdaughter is the daughter of a woman with whom the marriage was consummated, whether she was the daughter of a previous husband or a later husband.
It says in Fataawa Rasaa’il Samaahat ash-Shaykh Muhammad ibn Ibraaheem Aal ash-Shaykh (10/128):
The stepdaughter is the daughter of your wife with whom you have consummated the marriage, whether she is the daughter of a husband before you or of a husband after you. End quote.
The stepdaughter of a father or grandfather is not regarded as a mahram of  the sons or grandsons, so it is permissible for a man to marry the stepdaughter of his father and to marry the stepdaughter of his grandfather, because the stepdaughter is only a mahram [prohibited for marriage] for the one who consummated the marriage with her mother, as Allah, may he be exalted, says (interpretation of the meaning):
{Prohibited to you [for marriage] are … and your step-daughters under your guardianship [born] of your wives unto whom you have gone in} [an-Nisa’ 4:23].
As for his sons, they are not included among her mahrams. So it is permissible for them to marry the stepdaughter of their father.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) was asked: My father married a woman after my mother – may Allah have mercy on her – died, and this woman has daughters, but my father divorced this woman and did not have any children from her. Is it permissible for us to shake hands with her daughters and her daughters’ daughters?
He replied: They are not mahrams for you; rather they are mahrams for your father, because he married their mother, and they are stepdaughters if he consummated the marriage with their mother. As for you, you are strangers to them and you may marry them. They are not sisters or stepdaughters for you; Rather they are stepdaughters of your father.
To sum up: these girls are strangers to the sons of their mother’s husband." (Fataawa Noor ‘ala ad-Darb 20/290).
To sum up: it is permissible for you to marry that girl, because she is not a maternal aunt to you; rather she is the stepdaughter of your grandfather.
And Allah knows best.