Being married to a woman and her father’s ex-wife at the same time

It is permissible for a man to marry the ex-wife of his wife’s father, so long as that is not his wife’s mother. There is nothing wrong with marrying her even if her husband’s daughter is with him (is his wife), because there is no blood relationship between the two wives, i.e., between his first wife and her father’s ex-wife. What is forbidden is to be married to two sisters at the same time, or to be married to a woman and her maternal aunt or a woman and her paternal aunt at the same time. See question no. 22302. Anything other than that is permissible, because Allaah says, after mentioning those women whom one is forbidden to marry: 

“All others are lawful, provided you seek (them in marriage) with Mahr (bridal-money given by the husband to his wife at the time of marriage) from your property”

[al-Nisa’ 4:24 – interpretation of the meaning]

With regard to a mother and daughter: if the daughter is the wife, then then her mother becomes permanently forbidden for him to marry as soon as the marriage contract is completed. 

If the mother is the wife, then the matter is subject to further discussion: 

-If her husband consummated the marriage (i.e., has had intercourse with her), then the daughter is permanently forbidden for him.

-If he has not consummated the marriage, then the daughter is forbidden for him until he divorces the mother, because Allaah says, describing the women who are forbidden for marriage:

“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),”

[al-Nisa’ 4:23 – interpretation of the meaning]

From Kitaab Fataawa Islamiyyah, vol. 3, p. 134.