Sister living in tension after marriage.

My sister is married four and half years ago. She lives in Delhi. She is 31 yrs old and a housewife. She has a two year old baby. Her husband works in a govt company as an accounts manager. Her husband had affair with his ex girlfriend post marriage. When my sister came to know he promised he would not do again. But again he started talking to one female colleague in office who is also married. He used to talk to her for one hour in office but talked to his wife for 10-15 mins. When my sister came to know he promised again he will not do. Now he tells to my sister he does not talk to that colleague but my sister doubts him. She lives in tension. Do you have any solution to get away her tension? Can you tell who is specialist to consult?