How old are you Caveat? 
Very slowly, with great care, never in a hurry. First you start with the skirt…
What??
Oh, how do you address a nun?
Never mind. Carry on.
I just keep reading “undress” every time.
Nope. Setting aside the “Nuns of Christ” thing, a nun can be married (and have taken habits with her husband’s permission, of coruse this varies by order) or a widow. St Anne de Lestonac, founder of the order who owns my old school, was a widow.
I know nuns who prefer Mother, nuns who prefer Sister, nuns who go by Sister/Mother Lastname, nuns who use Sister/Mother Firstname, nuns who go by Firstname…
On first try I call them “Mother,” if they prefer something less highfalutin’ they’ll correct me but that way I never run afoul of one who prefers to be Mother’ed 
It’s funny how many squeak “ohmyGod, how did you know?” Gee, Mother, that grey skirt and the grey pullover and that metal cross pin are sort of a big giveaway.
- Huh. I’ve never even heard that Beatles song before.
Huh. I see from some quick research that married woman took vows in the early church, but which orders allow this today?
<sweetly>Bang, bang, shoot, shoot…</sweetly>
Although I’ll admit I skimmed right past it on first reading.
Don’t some (most? all?) orders have different habits (uniforms) for the Mother Superior?
Seconded. Marriage is a vocation and so is religious life, but it’s one or the other, or of course single life.
Do you mean St. Jeanne de Lestonnac? Which school did you go to? My husband & his sisters went to one in SoCal.
The Third Order Mendicants (a Franciscan group) allow married men or women, but depending on how you define it, they may or may not count as an order.
I was thinking that too. You can hope that you’re introduced. “This is Sister Mary Margaret.” Hurrah, problem solved.
If I wasn’t introduced I think I would do what others have suggested and simply say, “Hello, how would you like to be addressed? Sister?” "Do acquaintances usually call you “Sister Josephina”?
I’m sort of big on referring to people in the way that THEY wish to be referred to across the board.
It looks like I’m not getting the job anyway. 
Ah. Forgot about them. They are part of an order, but not what you think about when you consider nuns, brothers, or priests.
Come to think of it, the best way to describe the Third Order might be to call them a “monastic chaos”.
Sorry, yes. The one in Tudela (Spain). It’s known locally as “the nuns,” because the other nuns’ school has been there for a mere 50 years or so…