The Nun, released in 2018, is the fifth entry in The Conjuring horror film series and is set in the isolated grounds of foggy St. Carta. The movie takes the audience back to 1952 and is filled with historical religious horror elements. It follows the characters Father Burke and Sister Irene with a help of a local young guy called Frenchie, as they investigate a case of a nun suspected of taking her own life.
The abbey in the movie reveals itself to be a location of horrifying evil, with the characters encountering various demonic entities and experiencing disturbing visions. The movie is rich with hidden details, subliminal clues, and connections to other movies in The Conjuring universe.
In the 23rd minute of the movie, as the investigation of the abbey commences, Sister Irene notices a figure, shrouded in black, sitting behind them. When Sister Irene begins to speak in order to apologize for unannounced arrival, the figure asks them what do they want. Father Burke inquires if she is the Reverend Mother, to which she affirms. However, later in the film, Father Burke realizes that it is actually a corpse possessed by Valak, or the actual Nun demon.
So the conclusion would be yes, the Reverend Mother sitting in the black clothes was never actually alive, but the presence of the demon was speaking through the corpse of the actual Reverend Mother who was probably killed few weeks ago with all other nuns that prayed there for a salvation and in order to keep the devil locked inside the abbey.