Sweetie started out as a prostitute on The Strip, working for the Omertas in Gomorrah. Although she made more money, she hated the chems the Omertas forced their employees to use and decided to work for Pretty Sarah instead.
Her name is Sweetie because, in her words, she is "the sweetest piece you'll ever find outside the Strip."
A Bit of Slap and Tickle: For 300 caps, she can be hired as a prostitute by a Courier of either gender, not being particular about her customers, as Maude puts it. This strangely takes no in-game time.