Maybe a film about a brother and sister dealing with putting their elderly father in a nursing home isn't the best choice for a matinee with your mother (who's far from elderly, by the way) on a Saturday afternoon (though perhaps you'd enjoy having "This is a preview of my life. Are you trying to tell me something?" whispered in your ear while you're watching).
With Philip Seymour Hoffman and Laura Linney leading the cast, I'll admit I was predisposed to enjoy this movie before I even took off my coat and sat down, but it's difficult to imagine any other actors playing their characters because they inhabit the roles so completely, and Tamara Jenkins's script gives them so much emotional baggage and just enough backstory to play with.
Ultimately, Mama Cass actually enjoyed this movie because, in her words, "That's how life works," and that's exactly the poignant, bittersweet note that Jenkins hits while following two people still trying to put their lives together after an utterly dysfunctional upbringing. I realize that sounds mopey and depressing, but Jenkins never tries to make you feel sorry for her characters, or bogs down the story with unnecessary exposition explaining exactly what may have transpired between father and children (or brother and sister), nor does she try to create comedy, insert slapstick scenarios, or shoehorn cleverness where it doesn't belong.