Question Summary:
Marrying divorced sister in law

Question Detail:

 
Can a muslim man marry divorced sister-in-law and his first wife still exist and not divorced.
Please refer to the below link which is responded by Mufti Saifur Rahman Sahib , it is permissible. Please let me know your opinion on this.
http://www.muftisays.com/qa/question/1216/marrying-sisterinlaw-and-seeing-wife-before-burial.html

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
A person may not marry his wife’s sister as long as he is still married to his wife, or he has divorced his wife but she is still in her ‘iddah as it is impermissible to join to sister’s in one marriage. However, one may marry his brother’s widow or divorced wife after ‘iddah as long as there is no external reason of prohibition. For example, he already has four wives, or he is already married to her sister, etc.[1] 
And Allah Ta’āla Knows Best
Hisham Dawood
Student Darul Iftaa
Chicago, USA 
Checked and Approved by,
Mufti Ebrahim Desai.