In story mode, if you go back to the Border of the Fogs you'll meet Isabella. You need to rent the 1st house in Serpentine quarter first, then go back to Border of the Fogs, then the event will trigger. She'll offer to you to be her assistant, as she wants entry into Rome. Isabella is quite a good housekeeper, but lacking on the sexual skills. But it is quite a help to get her early in the game.
If you are playing as Johnny (or choose his portrait in custom start), a few days after you become a patrician, you will get a message that someone claiming to be your sister has arrived. Visit the Vatican with no slave to pick her up.