One of the things the novel is actually super-explicit about is how and where and characters learn what they know. (For the whole story, check out "Character Clues: Education.") Thackeray was interested in what gets passed down from one generation to the next and the ways in which traditions and culture are created. It was important for him to explore the ways in which characters are guided by their experiences and the people in their lives. He'd probably fall on the nurture side of the nature/nurture debate.
So, in another bit of nice symmetry, both Becky and Amelia have real and metaphorical mentors that teach them the ways of the world.
Becky's whole sad childhood story is one long lesson on how to please men, escape from crushing debts, and generally fake one's way through life:
She had the dismal precocity of poverty. Many a dun had she talked to, and turned away from her father's door; many a tradesman had she coaxed and wheedled into good-humour, and into the granting of one meal more. She sate commonly with her father, who was very proud of her wit, and heard the talk of many of his wild companions--often but ill-suited for a girl to hear. But she never had been a girl, she said; she had been a woman since she was eight years old. (2.15)
These amazing four sentences give us a pretty horrible window onto how quickly little Becky was forced to grow up under the pressures of her life with her dissolute father. And what do you make of that last sentence: "she had been a woman since she was eight years old"? Is there a hint of sexual abuse there? It's unclear. Later in life, Becky starts to learn a different tune under Lord Steyne, who is corrupt in body and soul. Becky is his protégé in high society, and he is her sugar daddy. What exactly does Steyne teach her? His cynical and worldly instruction brings out a more callous, less cautious side of her acquisitive and ambitious personality. What do you think – how would Becky have turned out differently if she hadn't met the dissipated, guilt-ridden monster Steyne?
Amelia is kind of a walking melodrama, so it's fitting that her metaphorical teacher is love. Again, Thackeray is pretty explicit about this:
We have talked of shift, self, and poverty, as those dismal instructors under whom poor Miss Becky Sharp got her education. Now, love was Miss Amelia Sedley's last tutoress, and it was amazing what progress our young lady made under that popular teacher. In the course of fifteen or eighteen months' daily and constant attention to this eminent finishing governess, what a deal of secrets Amelia learned [...] the sort of love that finished Amelia's education in the course of a year turned a good young girl into a good young woman--to be a good wife presently, when the happy time should come. (12.19-20)
There's nothing ambiguous here. Check out the clear reference to Becky and the poverty that taught her to fend for herself. This is the counterpoint to that, the novel is saying. Replace that horrible childhood with this romantic, dreamy, swoony one, and you've got Amelia's number.
After she gets some life experience under her belt, Amelia gets another guide to the world of adulthood: Peggy O'Dowd, the wife of George's regimental Major. Mrs. O'Dowd seems kind of loud and brash at first, but underneath she is totally devoted to personal responsibility and serving others. Amelia doesn't really become like her – she can't, since she lacks the inner energy and ability to self-motivate – but she does learn to get a little outside herself and commit herself to a cause other than loving dead George. What would she be like if she'd never met Mrs. O'Dowd?