Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Volume 18, Page 11, Fatwa No.4525, Question 2
Question: When the wife of a man dies and he [then] wants to get married to the sister [of the deceased wife], does he have to wait [for the passing of] the ‘iddah of the [deceased] woman?
I anticipate a fatwa regarding these issues, [and] should a man approach such situations or what?
Response: It is not necessary for the one whose wife has died and he wishes to marry her sister to wait for the [passing of] her ‘iddah, as there is no need for that.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.