I wasn’t over the moon about Lynn Shelton‘s Your Sister’s Sister (IFC Films, 6.15, theatres only) but I was moderately intrigued. It’s not bad. I thought it would get a Rotten Tomatoes rating between 70% and 80%, frankly, but it’s now at 89%. Obviously a sign of approval, but also, I think, of a “go easy” largesse that critics extend to earnest, low-key indie cheapies, especially those with a kind of John Cassevetes improvisational thing going on.

Your Sister’s Sister is one of those acting-class movies that are largely about a small cast (Mark Duplass, Emily Blunt and Rosemarie DeWitt) finding their way into emotional exposures and vulnerabilities and all-around discoveries that aren’t necessarily expressed or even explored in the script. It’s about “okay, this is the situation and here’s the basic plot…well, unless something else happens…and we all have a handle on our characters, of course, so let’s do this thing and see where it goes.”

And I was cruising along with this. Shelton and the cast are obviously trying to do “good” here, and I was rooting for them…yeah! But I ran into a character issue early on, and I couldn’t quite make it go away.

The film starts with a wake for the dead-and-buried brother of Jack (Mark Duplass ), who’s an unfocused, somewhat immature guy in his mid 30s. Various friends share gentle memories of the brother, who died a year earlier from his own hand, but Jack is pissed and unsettled about…well, a lot of things. But not about his best friend Iris (Emily Blunt). The movie tells us that (a) the closeness and trust they share is clearly based on their being strictly pals, and (b) Iris is one of the few solids in Jack’s life.

Note: the following has been divulged by critics all over so it’s not a spoiler, but there are whiners out there who will get upset anyway if I don’t say “spoiler” at this stage.

Reognizing Jack is off-balance, Iris urges him to take a break at her family’s vacation cottage on an island somewhere off the coast of Oregon or Washington, and he goes, “Yeah, okay, I guess so, whatever.” So he arrives at the cabin with his bicycle and backpack, and runs snack into Iris’s lesbian sister Hannah (Rosemarie DeWitt), or knocks on the door, rather, as she’s stepping out of the shower. She’s just broken up with her girlfriend of several years and has decided to use the cabin for a little meditation time. Ah-hah, awkward moment, okay…well, here we are.

Then they decide to relax and be friendly and enjoy a little tequila. Actually, they start slamming down shots. And then a randy vibe somehow creeps in and Hannah decides to forget about being gay and tumbles into bed with Jack. Just a little one-nighter so where’s the harm, right?

This isn’t the problem I spoke of. That occurs when Iris shows up at the cabin the next day and Jack gets all nervous — panicky, almost hyperventaliting — and starts lying his ass off so she won’t know what happened with Hannah. My question was “why?” As noted there’s not a hint of even the slightest erotic wannabe current between Jack and Iris, so if their friendship is truly solid and deep-rooted and telepathic then what’s the problem with Jack telling Iris, “This is going to sound weird but your sister and I got bombed last night and…uhm, we did it.”

Only a fundamentally dishonest and out-of-touch-with-himself guy with secret designs and/or longings for his “best friend” would go into lying and dodging spasms like Jack does. It seems to me that if Iris has been secretly in love with Jack all along and would therefore be devastated if she knew he’d slept with her sister, the movie should somehow convey this to us before Jack and Hannah do the nasty, etc. And it just struck me as phony. That’s all I have to say.

Postcript: Here’s an Indiewire-hosed chat about Your Sister’s Sister between Marshall Fine and Miami Herald critic Rene Rodriguez.