It's rare to find a film so heavy on the naturalistic realism that feels so effortless and unpretentious.
Your Sister's Sister opens at a remembrance gathering of friends for Tom, former boyfriend of Iris (Emily Blunt). When Tom's brother Jack (Mark Duplass) disrupts proceedings by discussing Tom's shortcomings rather than his virtues, he is hauled aside by Iris, his very good friend, who tells him that it's time to sort himself out and offers him her father's remote cabin for some time alone to think.
It's a scene that director Lynn Shelton (Humpday) chose to film last, though it appears first, for the simple reason that the two actors had had time by that stage to establish a real relationship - and it's a tactic that works wonderfully. The firm, if confused, bond between the two is charming, amusing and completely believable.
Your Sister's Sister feels unscripted - and it was, to a certain extent, with Shelton leaving her actors to adapt and even improvise lines within each scene.
But the relationships between the three characters seem so authentic that their conversations feel poignant without ever turning into self-indulgent waffle.
After a wonderful little scene in which the slightly past it Jack huffs and puffs his way to the cabin on a little red bicycle, he finds it already occupied - by Iris's sister Hannah (DeWitt).
This is a role that was originally intended for Rachel Weisz, who allegedly pulled out three days before filming. But DeWitt embodies the outwardly confident, inwardly crumbling Hannah to perfection, much as she did as the lost Bohemian artiste in the first series of Mad Men.
She is light-hearted, probing and fun - and most importantly for Jack, she is drunk, and, despite being a lesbian, trying to go to bed to him, after the two drown Hannah's sorrows about ending a stifling seven-year relationship in a hilarious scene involving two equally downcast souls and a big bottle of tequila.
The next morning they both hurry bleary-eyed back to their own beds when Iris decides to surprise Jack with a visit. But why should Jack care so much whether she finds out?
Shelton has a very light touch with dramatic tension and humour, as in one moment where adoring sister Iris and a frantic Jack simultaneously try to message the beleaguered Hannah behind each other's backs. Her naturally charming cast are made superb by patient directing and very shrewd editing. This is a film that has been very carefully packaged, despite its relaxed veneer.
The consequences of this fumbling night together are not entirely unpredictable, apart from a dramatic third act revelation that feels rather unnecessary and is possibly the only flaw in an otherwise exceptionally subtle drama.
But this is definitely a film where it is the journey and not the destination that counts. The enduring nature of sibling love is at its beginning and end, but we are interested not so much in whether the three characters can negotiate the events and moments but how they do so.
A great example of how much can be done with little budget and almost no time (it was filmed in just 12 days), Your Sister's Sister is billed as a romantic comedy but isn't anything like the brash rom coms that Hollywood makes so many of.
It's much, much better - and will make anyone with close families think keenly about them one way or the other. Especially sisters.