Rachel Weisz & Emily Blunt to Play Sisters Who Fight Over … Mark Duplass?

Lynn Shelton (“Humpday”) is directing the untitled, semi-improvised project

(Update 2 p.m. PST: "Humpday" star Mark Duplass has also joined the cast as a man who comes between Weisz and Blunt, reports Variety. Duplass currently stars on FX's "The League," and is represented by ICM.)

Oscar winner Rachel Weisz and Golden Globe winner Emily Blunt will play sisters in a semi-improvised movie from "Humpday" director Lynn Shelton, an individual familiar with the project has confirmed to TheWrap.

There's no logline yet for the currently untitled project, but I expect the actresses' mouth-watering pairing to top Blunt's sibling rivalry with Amy Adams in "Sunshine Cleaning."

Steven Schardt will produce the film, having co-produced "Humpday," which was one of my top-10 favorite movies of last year and also incorporated improvisation.

Production will last only a couple of weeks and is scheduled to begin later this month in Seattle after Weisz wraps a relatively minor role in Terrence Malick's next movie.

The "Mummy" star starts shooting Terence Davies' "The Deep Blue Sea" in November and will then segue to Fernando Meirelles' sex drama "360."  She recently completed Jim Sheridan's thriller "Dream House," and earned stellar reviews at the Toronto International Film Festival for her performance in "The Whistleblower."

Blunt recently signed on to star alongside Bruce Willis and Joseph Gordon-Levitt in Rian Johnson's "Looper." Oddly enough, Johnson's last film, "The Brothers Bloom," starred Weisz.

Blunt next stars in the Jack Black comedy "Gulliver's Travels," and will soon be seen opposite Matt Damon in "The Adjustment Bureau." The actress also voices one of the title characters of Disney's animated movie "Gnomeo and Juliet," and she's currently filming Lasse Hallstrom's adaptation of Paul Torday's novel "Salmon Fishing in the Yemen."

Shelton recently directed an episode of AMC's "Mad Men," and she just signed on to direct Focus Features' adaptation of Joshua Ferris' novel "Then We Came to the End," which Ted Hope and Anne Carey's company This Is That will produce. The filmmaker is represented by UTA and Industry Entertainment.

Weisz and Blunt are represented by CAA stateside, with Independent repping the former and Ken McReddie Associates reaping the latter overseas.

The beautiful duo's casting was first reported by Deadline.