If there’s one film trope that needs to be shot in the head, it’s the one in which a lesbian switches sexual sides in order to serve as the psychological healer/ self-actualizer for a guy, then conveniently goes away. (Chasing Amy [1997]: I’m lookin’ at you. Even worse: Three of Hearts [1993].)

The good news is that writer-director Lynne Shelton’s Your Sister’s Sister is not that film. The lesbian here is neither cardboard cutout nor fantasy object nor deus ex machina. She doesn’t go away. The bad news: once you peel away the tight dialogue, the great acting, and the enviable scenery, it’s not very far away from that trope.

Jack (Mark Duplass) is a mess. A year after his brother’s death, he still gets drunk at parties and insults everybody, including the memory of his brother. This behavior leads his best friend Iris (Emily Blunt), who used to date his brother and who seems perhaps a bit maternal toward Jack, to send him to her family’s cabin out on one of the San Juan Islands off the coast of Seattle for a solitary retreat to get his life back together.

Which reminds me to ask: why don’t my best friends have magical family cabins on islands? Do I not have need of self-actualizing retreats? Selfish friends.

But when he arrives at the house at night, the house already has a surprise inhabitant: Iris’s half-sister Hannah (Rosemarie DeWitt), who has just split up with her longtime girlfriend. Having finally come to terms with serious flaws in that relationship, Hannah’s almost as much at loose ends as Jack. So, naturally, they open a bottle of tequila and, in a drink-off, confess their sins to one another — relative strangers — across the table in an ugly, self-deprecating night of catharsis.

When the ordinarily withdrawn Hannah confesses that her partner’s callousness and infidelity has left her feeling unattractive, the very drunk Jack insists she’s perfectly hot. So much so that he’s willing to tell her — in very funny detail — because he knows her gayness won’t lead them to have sex. But in an uncharacteristic moment of risk-taking, she agrees to sex. What follows is one of the most unappealing, and utterly realistic, sex scenes between drunk people in the history of film. (You’ll be relieved to hear that this scene is also realistically short.)

So far so good, eh? There’s no risk of Hannah switching sides. Their sex is so impulsive and unpleasant (especially for her) that we know this is merely something to be embarrassed about down the road. But when Iris shows up the next morning and they decide to hide it from her, the narrative takes a strange twist toward ménage à trois.

I found the tale thus far to be charming, especially considering the director’s super-low budget (would you believe $125,000? did Shelton pay these actors at all?), the great scenery, and the endless appeal of stories about screwed-up individuals accidentally helping one another. I particularly liked the moments between the two half-sisters: the endearingly sweet, enthusiastic, little-sister Iris cuddling up to her older sister to chat in late-night hours.

If the dialogue seems a bit too clever in that CW Channel vein of overly scripted repartee, the ultimate stakes of the narrative aren’t very high. In fact, the tale gets a teensy bit mired in the “will they confess their drunken night of debauchery?” question, especially after Iris confesses to her sister that she thinks she’s in love with Jack, the brother of her now-dead ex-boyfriend. But hey, low stakes can be okay with me — not all movies have to be about world-changing ideas. Hell, this film is funny and the three individuals are appealing and good in dialogue with one another. Still, at midpoint Your Sister’s Sister feels a little bit like a short story stretched out over 90 minutes.

But when Iris learns the truth it all goes south — the three of them spin far apart from one another, with Iris blaming both of them for … some kind of perfidy she can’t articulate. (Warning: spoilers to follow, in which I reveal what I find distasteful about this portrayal of a lesbian onscreen!)

Unlike the horrors of homophobic films of the 1990s (again, Chasing Amy, one of the films I hate the most), this film doesn’t make Hannah’s gayness an issue or some kind of histrionic roadblock. Neither does this film pretend that a ménage à trois featuring a lesbian is some kind of risqué, challenging theme. Thank goodness for small favors as the progress of gay rights has rendered such narratives less viable.

Instead, it does something else I find annoying: in a big plot twist, Your Sister’s Sister turns Hannah into a woman so desperate to have a child that her drunken night of sex may, in fact, have been a ploy to get pregnant — a revelation that leads Jack to express huffy outrage as if she has somehow stolen his sperm, and which makes him leave the house on his bike, a form of physical exertion clearly new to his body. Most important, any possible romance between the two straight people appears dashed. The film turns into a straight-up love story between Jack and Iris that Hannah has ruined.

My problem isn’t simply that Jack would be a reluctant potential father, nor that a gay woman would want a child. My problem is that Hannah’s contrivance to get pregnant turns into the primary narrative dynamite destroying a weak and slightly annoying love story between the two straight people. Because that’s what this film ultimately becomes: a love story with a little ménage of lesbian thrown in, insofar as the lesbian functions as a reluctant partner in the threesome.

Honestly, this is where we’ve come? Straight man creates romantic problems for himself by sleeping with the sister of his true love interest? One could sum it up with a big “yuck.”

An ancillary problem is Mark Duplass, whom I quite liked in Safety Not Guaranteed (2012) as an unhinged inventor in search of a time-traveling companion (as well as Humpday [2009]). He’s good in independent comedy, as he’s capable of moments of real seriousness and has a nice talent to portray men slightly disgusted with themselves. But he doesn’t quite have what it takes to face off against actors as talented as Blunt and DeWitt. Nor does he appear capable of transforming from self-hating shlub to a man capable of responsibility or generosity.

Emily Blunt, on the other hand, is the film’s real center, primarily due to her ridiculous, approachable appeal. If I were in a sloppier mood I’d say she’s Hollywood’s new Sandra Bullock — one of those crazily beautiful women with a capacity to seem available even to shlubs — except that Blunt has an actorly range that has put her on a crazy upward spiral for several years now. She has appeared to great effect in everything from in top-shelf sci-fi bits like The Adjustment Bureau and Looper to period dramas like The Young Victoria to indie comedies like The Five-Year Engagement to romantic dramas like Salmon Fishing in the Yemen. The minute she appears onscreen, you know the quality of the film will tick up by a good ten points.

And when will Rosemarie DeWitt get her place in the sun? She’s excelled in small roles for a long while now — in Mad Men as Don’s freewheeling artist girlfriend in Season 1; in a bit role in Kenneth Lonergan’s Margaret; and most notably as Anne Hathaway’s soon-to-be-married sister in Rachel Getting Married. As I learned from JustMeMike‘s nice review of this film, she got slotted into this role late in the project after Rachel Weisz backed out due to scheduling conflicts. DeWitt does a great job as the depressive, withdrawn Hannah. And for those of you following my obsession with bringing back real women’s noses onscreen, just look at how DeWitt’s beautiful nose helps to establish her beauty and distinctiveness. God forbid she do a Jennifer Gray and scale it back to one of those ubiquitous, indistinguishable buttons.

Ultimately, Your Sister’s Sister feels like a halfway covenant — a reassurance that we no longer live with the homophobic narratives of the 90s, and therefore a gesture of good will from straights to gays; but no clear sign that writers know how to develop stories in which gay and straight characters coexist and inform one another’s lives in ways that feel true. Don’t get me wrong: it’s a film well worth watching, particularly for its first half and the nice dialogue. But it left me sad and dissatisfied in the end, fearful that Hollywood narratives of love between straight people will only be spiced up by the addition of tertiary gay characters.