A man married a woman then divorced her. He did not have any children from her. Then she married another man and gave birth to a girl. Is this girl considered a step-daughter to her mothers' ex-husband and hence, one of his Mahrams (unmarriageable relatives)?
If the woman's first husband divorced her after he had consummated the marriage, then her daughter from the second husband is considered a stepdaughter of the first husband and hence, it is unlawful for him to marry her according to Allah's saying (Exalted be He):
Forbidden to you (for marriage) are: your mothers, your daughters
to His saying:
...your step daughters under your guardianship, born of your wives to whom you have gone in
(Part No. 18; Page No. 215) Saying that the stipulation of the guardianship of the step-father to the stepdaughter is for the predominant case only, has no effect on the ruling. If the first husband divorced her before the consummation of the marriage, she does not have the same ruling as a stepdaughter and it is permissible for him to marry her because the Qur'anic verse prohibits a step-father from marrying his step-daughter if he had consummated the marriage with her mother. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.