As she proved with 2009’s “Humpday,” director Lynn Shelton is a talented filmmaker not afraid to include LGBT themes in her work. Her latest is “Your Sister’s Sister,” which has a prominent lesbian theme, and — like “Humpday” — is worth seeing even if the gay angle is a bit problematic.

As the film opens, Jack (Mark Duplass) is mourning the death of his brother Tom, a year later. After a friend waxes eloquently about Tom at a gathering, Jack stands and speaks the warts-and-all truth about the deceased and it’s clear Jack is still a wreck. His best friend is Iris (Emily Blunt), who was Tom’s lover. Seeing Jack is an emotional mess, she suggests he go to her family cabin on an island in the Pacific Northwest that is supposedly empty. “It has no internet, no TV… maybe a few forks,” she promises.

When he arrives he discovers he is not alone after all. Iris’ half-sister, Hannah (Rosemarie DeWitt), is there. She is a lesbian who has just broken up with her girlfriend after seven years. She is also a vegan whose food choices (such as dehydrated banana pieces) provide some of the film’s funnier moments. The two share a drunken late night and reveal some truths.

The first 30 minutes of the movie move quickly and are funny and tart, with three well-developed characters. Then something happens between Iris and Jack, however, that throws a wrench into the situation. It’s clear that what happens between the two is a mistake and it becomes evident as well that there could have been ulterior motives for both of them. It’s here that the film moves from being a sharp comedy to something a little more dramatic and safe. Not surprisingly, Iris comes to visit at the cabin as well and the relationships between all three become more complicated and tense.

Comparisons can be made to the “The Kids Are All Right,” though this film doesn’t vilify its  male character at the end — and what happens between Iris and Jack  is a wee bit more believable, thanks to a lot of tequila and need. 

In “Humpday,” two straight friends spent the entire movie convincing us and themselves they were secure enough in their friendship and masculinity to have sex on camera to win an amateur porn festival, then freaked out when it was time to do the deed and decided against it. Like “Humpday,” “Your Sister’s Sister” might be viewed a little differently by straight and LGBT audiences. Overall, “Your Sister’s Sister” is much better than “Humpday,” even if — groan — after establishing Hannah as a complicated, fully developed person, the film takes a U-turn and has her do something that strains credibility.

But what Shelton does get right is her cast — she loves her characters, even when they do stupid things. These three performers are the only people in the entire movie and luckily, they make the ride enjoyable.

Blunt has been all over the big screen this year and she is one of our most warm, likable actresses. She is also a great comedienne. DeWitt is downright luminous. Best known for her supporting work in “Rachel’s Getting Married,” “United States of Tara,” and “Mad Men,” she does some of her best work here. As “Rachel” proved, DeWitt knows how to play a sister. She and Blunt are very effective in that aspect, and their sibling rivalry is believable. 

Duplass (from “Humpday”) is the lesser known of the three but he is a nice choice for Jack, with a dry sense of humor yet an undeniable pain underneath.

“Your Sister’s Sister” is smartly written and has some droll dialogue to match its acting. Let’s just hope that at some point Shelton can get it all right, including the LGBT component. She’s getting closer and closer to great work.

Top photo: ‘Your Sister’s Sister’ stars Rosemarie DeWitt (left)  as Hannah, a lesbian who strikes up a friendship with her sister’s best friend, Jack. Jack’s best friend, Iris, played by Emily Blunt (right), joins the mix for a poignant film of laughs about friendship and family. (Publicity photo)