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.

It is not permissible in Islam to marry two sisters at the same time, because they are sisters, regardless of whether they share the same father and mother, or they have only one parent in common.