Nyssa's mother was a Russian Jew in the late 1700s. (I don't think she was ever named in the story.) Ra's had business in Russia, wanted a bedwarmer for the freezing winter, didn't realize he'd gotten the woman pregnant before he left her.
Nyssa was 20-something when her mother died of disease. She went looking for Ra's, eventually found him, and became his apprentice for a century or so.
I've got the story Nyssa originally appeared in (Batman: Death and the Maidens), if you'd like me to post some scans from that.