It was only a matter of time before the sisters were found out.
Even after Narcissa was married, she stole secret moments with her sister. They were her only joy in a life that had become a string of parties at which she was forced to appear as Lucius Malfoy's precious trophy of a bride. When she was in Bellatrix's arms, Narcissa could forget that most of the time, she was thought of as nothing more than a pretty face, something that Lucius had won.
But they were found at last.
They had been brought before their parents to be shamed, and Cygnus and Druella went on and on about unnatural desires and how they were blights upon the family tree who did not deserve to be called Blacks.
Bellatrix and Narcissa took the scolding until their parents had clamed themselves, then they were ordered never to tell anyone, never to let anyone else know about what they had done, and the whole matter became another of the myriad family secrets that were hidden within the walls of Black Manor.