On Nina Dobrev’s last TV gig, she was dropped into a family of vampires.

This time around, she’s got an annoying little sister. If you see her, be diplomatic and don’t ask which is worse.

Fam, which premieres at 9:30 p.m. ET Thursday on CBS, has Dobrev (top) playing Clem, a successful young professional who is just getting engaged to the most wonderful man in the world, Nick (Tone Bell, top).

That means she also inherits the world’s most perfect in-laws: Nick’s parents Walt (Brian Stokes Mitchell) and Rose (Sheryl Lee Ralph).

So her life is way too perfect to survive even the first episode, and sure enough, Clem and Nick return from his parents’ place to find trouble sitting on their living room couch.

Enter Shannon (Odessa Adlon, top), who casually reveals, in response to Clem’s question, that she got in by picking the lock.

Hi, Sis. Well, half-Sis.

Shannon is 16, and she would be a tricky character to play if Fam weren’t a sitcom. She’s dumb as a doornail when it comes to a lot of common-sense things that most people know by the age of 8, and at the same time, she’s got excellent street instincts. It’s hard to slip things by her.

In a sitcom, dichotomies like that don’t have to be explained. Whichever way a joke works, smart or dumb, it requires no explanation why that’s how she’s playing it.

That said, Adlon quickly becomes engaging. She’s the character you most want to hear speaking the show’s next line of dialogue.

Take the fact she dropped out of school. When Clem asks her how she did that, she says she just stopped going.

She’s also dating a drug dealer, and she carries around a snack bag full of marijuana-infused nuts. No prizes for guessing that pretty soon that bag of nuts gets gobbled by someone other than Shannon.

This is, after all, a sitcom, and once you’ve set up that situation, it’s way too inviting not to exploit it.

The show’s baseline premise, though, requires one further twist. It seems Clem’s and Shannon’s father Freddy (Gary Cole) is a classic deadbeat dad. He holds a fulltime job as a homicide detective. He’s just totally indifferent to the kids, which explains why Clem, a few years earlier, behaved just the way Shannon is behaving now.

At first, Clem tries to talk Freddy into taking Shannon back and maybe steering her toward something like school. But Clem then admits that Freddy, who’s played with hilarious deadpan disinterest by Cole, isn’t a promising option. She invites Shannon to live with her and Nick, an invitation with which Mr. Perfect, Nick, is totally fine.

We like Nick and Clem well enough, so we want them to be happy and succeed in their mission. Whether they can do both is, of course, the question that will create the dramatic tension of Fam, which is about exactly what the title suggests: the nature of family.

Toward that end, Fam starts on a fairly modest building block. But when a show like Mom can succeed with an equally flimsy setup, perhaps we’ve entered a stretch where we will watch a small story if the wild card is untethered enough.

On that score, with Shannon, Fam has a shot.