A man marrying the daughter of his father's former wife
After my father's death, my step-mother married my paternal uncle and begot him a daughter. Is it permissible for my son to marry this girl although her mother is his grandfather's wife?
If your father's ex-wife that your uncle married, is not your mother, it will be permissible for your son to marry your uncle's daughter as what is forbidden for a Muslim is to marry his stepmother or his grandfather's wife for Allah (Exalted be He) says,
And marry not women whom your fathers married, except what has already passed
Therefore, prohibition includes only women whom your father married but does not include the father's stepdaughters or their sisters. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.