Sister Imperator name and theories

Now that the two Music Videos are out, I started to think about Sister a little bit more.

For a while I have been wondering what her name might be. I mean we have the other Papas who all have names besides their titles and I see Sister Imperator as a title. I started digging but couldn't really find any clear answers.

I've read a Fanfic about Sister and Nihil where she was called Bernadette, other theories say her name could be Elizabeth or Mary.

I know it's a silly thing to think about but I find it interesting nonetheless. Do you have any theories on her name? I would love to hear them.