Friends Carson (Lena Dunham, left) and Jenny (Anna Kendrick) play with the real star of “Happy Christmas,” baby Jude (Jude Swanberg). (Magnolia Pictures/Magnolia Pictures)

When an aimless 20- or 30-something moves in with a relative, conflicts and/or awkwardness inevitably ensue. That’s what contemporary indie films have taught us, anyway. Such movies as “Lars and the Real Girl,” “Our Idiot Brother,” “Jeff, Who Lives at Home” and many others have conditioned audiences to expect the worst, or at least the weird, whenever an arrested adolescent decides to shack up with a parent or sibling.

In “Happy Christmas,” then, it’s no surprise when things get messy shortly after Jenny (Anna Kendrick) starts bunking in the tiki-themed basement located in the home of her brother, Jeff (director Joe Swanberg); his wife, Kelly (Melanie Lynskey); and their young son (Jude Swanberg, perhaps the finest baby actor of all time).

But what is often surprising in this entertaining and fluidly acted portrait of females in flux is the specific way things get messy. Sure, Jenny engages in some predictable unreliable-millennial behavior: She stays out too late, drinks too much and frustrates her sibling and sister-in-law, who want to raise their 2-year-old in a peaceful environment. But most moviegoers probably won’t anticipate that Jenny and Kelly will eventually start bonding while penning a work of erotic, “Fifty Shades of Grey”-esque fiction.

As in his previous low-budget, mumblecore efforts, including “Hannah Takes the Stairs” and last year’s astute “Drinking Buddies,” Swanberg’s filmmaking style here is free-wheeling and naturalistic. Though some scenes were sketched out before cameras rolled, the dialogue in “Happy Christmas” is largely improvised, with much of the action unfolding in Swanberg’s real-life Chicago home. That approach gives the movie an easygoing, organically developing quality that makes it believable. Yet with a running time of 82 minutes, the story is concise enough to ensure that all that meandering doesn’t wear out its welcome.

Such a loose structure requires the actors to carry much of the day, ad-libbing in a way that feels spontaneous yet purposeful. This cast has no problem meeting that challenge. Kendrick, who worked with Swanberg in “Drinking Buddies,” measures exactly the right doses of irresponsibility, shame and resilient optimism into her portrayal of Jenny. And Lynskey — a consistently excellent and underrated actress — is an authentic, endearing ball of maternal doubt as Kelly, a creatively blocked writer still adjusting to the aftershocks of parenthood. Whenever Swanberg puts Kendrick and Lynskey in a room together — and especially when he adds Lena Dunham to the mix as Jenny’s slightly more together friend, Carson — their conversations park in amusing, revealing spaces.

“I’m just trying to create a world in which you can have it all,” Carson tells Kelly during a discussion of the pressures of stay-at-home motherhood, prompting Kelly to reply, with the weary wisdom that only a full-time diaper-changer possesses: “Having it all is a very dangerous concept because it means I just have to do everything.”

Were these exchanges ripped, verbatim, from the back-and-forth between you and your girlfriends at least week’s book club meeting? No. But they sound like they could have been. In fact, the one mistake Swanberg makes is failing to focus even more closely on the dynamic between Jenny and Kelly; the side plot about a tentative romance between Jenny and a pot-dealing babysitter (Mark Webber), for example, easily could have been jettisoned in order to give the sisters-in-law relationship more screen time.

But speaking of babysitters: Oh my goodness, can we please talk about the baby in this movie? If the Academy of Motion Picture Arts and Sciences decides to add an Oscar category called Best Performance by a Toddler in an Indie Film Directed by His Own Father, Jude Swanberg would easily smoke any precocious competition.

No doubt buoyed by the presence of his dad and the familiarity of his surroundings, Little Jude reacts with vivid and engaged spontaneity to everything around him. Each time he giggles with baby-swinging glee, mimics his father’s gestures or shoves fistfuls of Cheerios into his mouth, his castmates mirror his delight. A director can’t orchestrate moments like that, even when he’s working with his own kid. They just happen, the way life does and the way so many of the relatable, lovely moments in “Happy Christmas” do, too.

Chaney is a freelance writer.

★ ★ ★

R. At West End Cinema. Contains strong language, drug use and some sexual content. 82 minutes.