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.