Nuns are called sisters, but are they actually called "my sisters"? Anyway, to my ear this sounds like someone emphasizing the compliment by going for excessively formal, kind of same as someone using a full name even while being on close terms..."Miss N.N., how beautiful you are today!"
Yes, French Catholics address nuns with "ma soeur" and priests with "mon père".
Non-Catholics address nuns with "soeur Anne" (first name) and priests "père Dupont" (surname)
Soldiers address their officers with "mon capitaine, mon commandant, etc"
Civilians address officers with "capitaine, commandant, etc".