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That good old German term kitsch perfectly describes “Keeping Up With the Steins,” the feature directing debut of Scott Marshall (son of Garry), which–like the recent “When Do We Eat”–mines the “My Big Fat Jewish Wedding” vein of supposedly warm ethnic humor, but with a Jewish accent. The only difference is that instead of a wedding ceremony or a Passover seder, the affair this time around is a bar mitzvah. And the celebration once again proves pretty much insufferable.

The juvenile centerpiece of the plot is young Benjamin Fiedler (Daryl Sabara), the son of Hollywood agent Adam (Jeremy Piven) and his doting wife (Jamie Gertz). After attending an absurdly extravagant cruise-ship bar mitzvah with a Titanic theme hosted for his son by his ex-partner Arnie Stein (Larry Miller), Arnie determines to outdo it, scheduling a bash at Dodger Stadium while failing to notice Benjamin’s wariness about the whole affair, especially since he’s failed to master the necessary chants and has gotten no real understanding of the ceremony from the family’s stand-offish rabbi (Richard Benjamin). In an attempt to deflect his father’s attention from the arrangements, Benjamin sends an invitation to his grandfather Irwin (Garry Marshall), who abandoned Adam and his mother Rose (Doris Roberts) many years before and whom his son hasn’t forgiven for the desertion. Worse, he makes sure that the invitation is for a full week earlier than the ceremony.

So Irwin shows up, a gregarious aging hippie driving around in an RV with a much younger wife who calls herself Sacred Feather (Darryl Hannah). The family receives the duo warmly–even Rose is surprisingly welcoming–except for the furious Adam, who can’t release his anger. Irwin develops a particular rapport with young Benjamin, using his savvy to help the boy with both his preparations for the bar mitzvah and the kid’s tongue-tied infatuation with a with a cute but snooty classmate. And indeed the boy matures so much that he not only gets through the ceremony gracefully, but also jettisons the plans for the big party in favor of a back-yard affair at home, recognizes how shallow the pretty girl is, and–most importantly–brings his father and grandfather back together.

It should be obvious from this that Mark Zakarin’s script is the purest corn, and it’s played without restraint. Marshall, in particular, goes full-throttle, even allowing himself a nude sequence that provides the sort of harmless naughtiness old ladies in the audience will giggle over in mock embarrassment. But his performance is at least jovial. Piven, on the other hand, is so maniacally unpleasant that you can only hope he’ll stay off-screen as long as possible, and Miller matches him in mirthless intensity. The distaff side of the equation–Gertz, Hannah and Roberts–is much better, far more laid-back and likable. As for young Sabara, he’s as amateurish as most of the kids on network sitcoms, but given the nature of the Fiedler family one can’t help rooting for him anyway. And Benjamin and Cheryl Hines, as a stressed-out event planner,

Technically “Keeping Up With the Steins” is pretty mediocre, typical of lightweight independent product. But it’s visually inoffensive. It’s the schmaltzy content that’s the problem. Bar mitzvah or not, it’s still awfully juvenile.