"Mother"/"Mother Superior" -is- a valid way of addressing a 'head' nun, though it is rare to use it nowadays. They usually prefer "Sister [insert name]" now.
Source: Worked part-time in a Catholic diocese office for awhile.
@LooumiiC1nt
@Meybi
Both are Alternate History slang, so apologies. There are significant crossover elements in storytelling between Isekai and "Alien Space Bat"-style AltHist, so its fairly well used elsewhere. The Alternate History and SpaceBattles forums' fiction communities notably.
ISOT...