So how many really good movies can you name about brother-sister relationships? And what if you limit your list to only those about adult siblings? Don't be surprised if your list is empty, or at least close to it. In the future, any such list will have to start with YOU CAN COUNT ON ME, the winner for Best Picture and Best Screenplay at this year's Sundance Film Festival. Written and directed by Ken Lonergan (the writer of last year's hit comedy ANALYZE THIS), YOU CAN COUNT ON ME is a rich and witty drama whose centerpiece is a sibling relationship.

Laura Linney (Truman's wife in THE TRUMAN SHOW) stars as the sister, Sammy. Sammy tries to be a level-headed mother to her 8-year-old, Rudy (Rory Culkin). Her ex is a permanently AWOL father to Rudy, who has never seen Rudy Senior. Her brother, Terry (Mark Ruffalo), has come for a visit. In a poignant restaurant scene, she shows that she is there for him, but he is there for money. It momentarily breaks her heart.

But this isn't one of those dysfunctional families movies, even if their relationship does have its difficult moments. This is a story of love, not hate. The chemistry between them and between Terry and Rudy is remarkable and genuine.

Sammy has had a modestly successful life, but Terry is a screw-up, although he claims, as if he doesn't believe it, that "I'm not the kind of guy that everyone says I am." They grew up in the small backwoods town of Scottsville, where everyone knows everyone else. It is a "dull, narrow town, full of dull, narrow people," Terry tells Rudy in a long put-down of Scottsville. "What are you talking about," Rudy asks him with soulful, confused eyes. "I have no idea," Terry confesses in a bit of self-reflection.

Their parents died in a car accident when Sammy and Terry were young. In a brief prologue, we witness the accident almost matter-of-factly. The beauty of the delicate script is the way tragedy is consistently downplayed and that key incidents are merely sketched out. The writer allows us to use our imagination to fill in the details. And the direction is done with great subtly and finesse so that there isn't an overacted or schmaltzy moment in the production. The result is a picture so authentic that it has the audience hitting the ground running. Only a few minutes into it, and we feel like we've known and cared about these people all of our lives.

Sammy laments, "I wish Mom was here," when talking to her brother about how to handle a troublesome issue. Anyone whose mother has died certainly thinks this and often. Linney delivers this line with heart-felt but restrained emotions, making it feel as genuine as if she were talking about her own dead mother.

Another part of the plot concerns Brian (Matthew Broderick, ELECTION), Sammy's new manager at the bank. His is an anal-retentive type, who admits that "I like paperwork." He wants daily time cards and a conservative color palette for the bank's PC displays. Brian and Sammy are a combustible mix, but rather than getting fired, as it looks like she is destined to be, Sammy ends up sharing fireworks in bed with her boss instead. This affair shocks and delights her at the same time. In Linney's best bit of acting, she lets out a small series of nervous, joyful laughs in the car as she sits there by herself after their first fling. (The writer gives us a few glimpses into what may or may not be Brian's unhappy married life with his pregnant wife.)

Besides writing and directing, Lonergan also plays the part of Sammy's laid back priest, the funniest character in the movie. She wants him to tell her that she will burn in the flames of eternal damnation for her sins, but he has a more flexible moral code, which allows for continual redemption.

Linney, with some of the spunk that Holly Hunter demonstrated in BROADCAST NEWS, delivers a delicately nuanced and wonderfully appealing performance that deserves to be remembered at Oscar time. But you want to see this movie now and not wait until the nominations come out to remind you of the great movies that you missed.

YOU CAN COUNT ON ME runs 1:49. It is rated R for language, some drug use and a scene of sexuality and would be acceptable for most teenagers.