Family Drama, Reforming the Rake
Pamela makes use of a kind of psychological realism that was unprecedented in the larger genre of the novel up to that point. But just because it's realistic doesn't mean it's realism, and Pamela predates the genre of literary realism by, oh, a century or so. (But it was hugely influential.)
Mostly, the novel concerned with two families—Mr. B's and Pamela's—which is why we think "family drama" is one of the best fits. We're also going to put a plug in for a little genre we like to call "Reforming the Rake," since Pamela basically invented the genre.
Have you read Jane Eyre? Okay, then how about Twilight? Whether you have or not, we guarantee you'll recognize the plot: bad boy is bad, and possibly sad as well. Women are nothing but playthings to him, until he meets an innocent girl who saves him from himself with the power of her virtue slash general awesomeness.
Been there, done that, right? Well, not before Pamela.