Is your summer mind clouded with pressing questions: Is the Higgs Boson/God particle really God? Was Mitt Romney working for Baine in 2002 or not? Have they really found Amelia Earhart? Will there be a fourth volume of “Fifty Shades of Grey?” Relax. A new movie is about to open that will give you a break, let you breathe without thinking, and in the bargain, provide some laughs and two nice performances.

It starts with a party at Iris’s (Emily Blunt) summer cabin in the woods. That’s a good sign. We like parties. Sadly, this one turns out to be a eulogy gathering in which all participants say happy things about the deceased, that is, but for the dead Tom’s brother Jack (Mark Duplass) who doesn’t have a lot of nice things to say about his sibling. Bummer.

Those of us who remember the 80s have a template for this kind of story: Larry Kasdan’s 1983 “The Big Chill,” that introduced a spate of stars that have now all moved on to the small screen.

So, here we are in Iris’s hideaway overlooking the Pacific Northwest’s San Juan Islands. (I’ve been to the San Juan Islands. You don’t want your funeral held there.) Before long there will be two more characters, the third, a woman, the fourth will arrive later and blow everything up.

Mercifully, the party is over quickly, and we get to the point of the film. Our hero Jack finds himself adrift in life after the loss of his brother, and our heroine, Iris, who may have long held a crush on our Jack. She feels a maternal, or maybe just sexual, pull to guide him back onto to the working grid. She offers him the use of her cabin while she’s away, so he can focus on his navel or the fog of Puget Sound and find nirvana, or at least a link to world peace.

Jack arrives on a foggy night and sees a light. Peering through the window, he spies a comely maiden taking a shower. Hearing sounds outside, our maiden rushes out and chops Jack with an oar. Fear not. Jack lives. The maiden takes him in from the cold, and we discover that this is Hannah (a very good Rosemarie DeWitt,) Iris’s vegan lesbian sister. All of that doesn’t come out at once. The lesbian part first, and then the vegan part over a couple of meals spoiled by carnivore sister Iris.

“Your Sister’s Sister” turns out to be very educational. We learn that vegans can not do dairy or meat, but feel quite free to inhale prodigious amounts of tequila. And despite a pox on cheeseburgers, sex with one’s sister’s boyfriend is okay.

So, Jack and Hannah tumble into Iris’s bed after a long search for a condom. Keep your eye on that condom. Remember what I said about a fourth character? That would be the condom.

What do you think happens the next morning? You’re so smart, how did you know? Iris returns home ready to spill her own secret on the table. Of course, if she didn’t, this would be called “Two On a Match.”

Jack swears Hannah to secrecy, and so the trio spends much of the rest of the movie blending harmoniously with each other and nature. They cook and drink, reveal nuggets of background stories, and sit on the time bomb that ticks away in the background. When it is finally revealed, it’s sort of anti-climatic until the fourth character, that mysterious condom, comes into play. OMG!

In all, “Your Sister’s Sister” is one of those independent summer films that floats around the humid haze like dandelion floss. Apparently, improvisation is back in style again. Some can work, some can’t. This trio pulls it off. The picture is well made and raised above mediocrity by two really good actors, Rosemarie DeWitt (“Madmen,” “The Company Men”) a strong actor with layers of sophistication and sensual smoke curling around her eyes. Duplass is a mediocre actor working with two powerful women which only heightens his weaknesses. Duplass seems to work a lot these days, so maybe I’m wrong, but I get the feeling they couldn’t afford Seth Rogen.

And then there is Blunt, she of the watery eyes and trembling lips, who is pretty much the same in all of her films, the way all the great stars of the 40s were pretty much the same.

It worked then. It works now.

J.P. Devine is a former stage and screen actor.