Casting Amy Poehler and Tina Fey as sisters is an irresistible idea that should have been resisted until a better script than “Sisters” came along. The two are among the best duos currently working in comedy (not to mention their individual brilliance), and fans have been waiting seven years for a followup to their good-but-not-great “Baby Mama.” Tina and Amy are magical as Golden Globe hosts and wherever else they pop up together. So why aren’t they killing us in movies, too?

Let me tell you the premise of “Sisters,” and you see if you can spot the red flag. The Ellis sisters — do-gooder liberal Maura (Poehler) and flaky trainwreck Kate (Fey) — learn that their parents are selling the Orlando house they grew up in, and though they’re in their 40s and living on their own, the girls haven’t quite left the nest emotionally. Recalling the glory days of their high school parties, they throw one last shindig in the old house before the new owners arrive.

Did you catch the warning sign? It’s having Fey play the slutty, irresponsible one. Poehler has proven adept at playing both Type A personalities (like she did on “Parks and Recreation”) and hot messes (like she did in “Baby Mama). But Fey — always a better writer than performer (Poehler is the opposite) — hasn’t demonstrated that kind of range, and she’s never quite convincing in “Sisters.”

It doesn’t help that Kate is written so inconsistently. (It’s the first produced screenplay by the very funny “SNL” writer Paula Pell.) An unemployed cosmetologist with a teenage daughter, Kate is supposed to be coarse, reckless, and kind of dumb. But she keeps saying things that are smart and self-aware — you know, the sort of things Tina Fey would say. It’s like Tina is trapped inside Kate, trying to claw her way out.

Casting issues aside, the film has problems but not insurmountable ones (like the fact that all of the girls’ high school classmates still live in town and can come to a party on one day’s notice), and it’s occasionally hilarious. Unsurprisingly, it’s at its best when Kate and Maura are together, teaming up against their parents (Dianne Wiest and James Brolin), flirting with a handsome neighbor (Ike Barinholtz), or vetting potential party guests. Though they are opposites, the sisters are not enemies (until the inevitable third-act quarrel), and their camaraderie as they face enemies together is appealing.

Our review of “Sisters” from the Movie B.S. with Bayer and Snider podcast:

Download audio

Maura’s romantic subplot with the handsome neighbor slows things down, and it culminates in a truly awful scene of physical comedy that is almost Sandlerian in its idiocy. (The director, Jason Moore, has one other film to his credit: “Pitch Perfect.”) On the other hand, the party is like a series of mini-sketches, with many bright spots provided by the likes of Samantha Bee, Kate McKinnon, Matt Oberg, Greta Lee, and wrestler-turned-actor John Cena (whose flirtation with Fey is a highlight). Bobby Moynihan’s performance as a former class clown whose manic sense of humor never matured past high school isn’t very funny, but Moynihan’s commitment sells it. I wish the same could be said for Maya Rudolph, who’s wasted as an old rival of Kate’s.

“Sisters” fails to show why so many people love Tina & Amy, but it’s not enough of a buzzkill to make anyone relinquish their membership in the fan club. In fact, I suspect someone with no prior experience with these two would enjoy the movie more than those of us who are invested. It isn’t bad, after all (though aspects of it are); it simply isn’t as good as everyone involved is capable of. I feel like a parent here: I’m not mad, just disappointed.

C+ (1 hr., 57 min.; R, a lot of profanity and vulgarity, some sexual innuendo.)