Portuguese irmã, Spanish hermana and Catalan germana (as their masculine counterparts) come from the word germanus, germana, an adjective that the Romans began to add to the words frater and soror (brother and sister) to denote that they were talking about biological siblings, not adopted ones (adoption, even of adults, was very common in Rome). Hence, frater germanus and soror germana, that those languages reduced to the second element.
Just as in English "Sister" used to be the correct official form of address to any nurse (who was not in charge of a ward, that was "Matron", from the Latin for "mother:). Now that nurses can be men, the titles have changed. I believe that the origin of this lies in the fact that the first hospitals were run by nuns; the religious titles were preserved, even for secular nurses.