Question Summary:
It is prohibited to fantasize about another woman while being with one’s wife

Question Detail:

 
Can you tell me if a person had to fantasize about have sexual intercouse with ones mother-in-law and sister-in-law , would this break the marriage with his wife ?
 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh
It is prohibited to fantasize about another woman while being with one’s wife. This is a major sin and disrespectful to one’s wife. However, fantasizing about another woman during intercourse does not break one’s marriage and does not constitute Hurmah Musaharah (incest).
 
And Allah Ta’āla Knows Best
 
Ismail Desai,
Darul Iftaa
 
Checked and Approved,
Mufti Ebrahim Desai.