From the Observer's Guide to Popes, Cardinals, Bishops, Priests and Religious

It was recently brought to my attention by a Religious Sister, that she was not a nun. Upon looking it up, I discovered that "monk" and "nun" refer only to cloistered religious, while "friar" (or "brother") and "sister" refer to religious who live an active vocation in the world of service to the needy, sick, poor, and uneducated. The confusion arises because, just as we address all priests as "father," so we address all male religious (who are not priests) as "brother," and all female religious (who are not abbesses) as "sister."

That's right — you weren't taught by nuns, you were taught by sisters!