Being a guru, Brihaspati had sex with his younger brother's wife and yet why did the devas let him be their guru?
This is is mentioned in Devi Bhagavata Purana here.
Did Brihaspati suffer any consequences because of this act?
The Matsya Purana too tells us a strange tale about Brihaspati. According to this story, Brihaspati once met his elder brother Utathya's wife Mamata in solitude. Tempted by her beauty, Brihaspati sought sex with her. And when she refused, he took her by force though she was pregnant at that time.