Marrying the sister of his ex-wife
Question

                Is it permissible for a man to marry the sister of his former wife, when the ‘iddah of the former wife has ended and if the first wife is still alive? Because the prohibition applies to being married to two sisters at the same time, and the former wife is alive.
Praise be to Allah.Yes, it is permissible to marry the sister of 
    one's former wife, on condition that the ‘iddah of the first wife has ended. The evidence for that is the verse (interpretation of the meaning): 
“Forbidden to you (for marriage) are: … and two sisters in wedlock at the same time”
[al-Nisa’ 4:23]
It was narrated that ‘Ubaydah al-Salmaani 
    said: “The Sahaabah did not agree on anything so unanimously as they agreed upon four (rak’ahs of Sunnah prayer) before Zuhr, and that a woman 
    should not be taken in marriage during the ‘iddah of her sister.” 
The prohibition applies when the marriage is 
    still in effect, but now the relationship with the first wife has ended through divorce (talaaq). 
And Allaah knows best. 
See al-Mughni, 7/68-69.