Sister-in-law had sex with brother-in-law's friend

Sister-in-law had sex with Deora's friend and brother-in-law's friend told this to the brother-in-law, when the brother-in-law came to ask his sister-in-law why did you have sex with my friend, initially the sister-in-law said that I have not had sex but later the sister-in-law got scared and she said please do not tell this to your brother-in-law instead of telling the matter. The brother-in-law also had sex with his sister-in-law