A dysfunctional family gathering for a special occasion (be it a wedding or a funeral) has long served as a ripe dramatic backdrop for a reason. What better excuse to bring together a motley crew of characters who know exactly how best to push one another’s buttons and dredge up long-simmering resentments in the process? If Kristin Scott Thomas ’ “My Mother’s Wedding” never quite reaches the heights of that well-worn genre, it is not for lack of trying. Equal parts Anton Chekhov and Richard Curtis, her directorial debut ends up being a sweet if rather modest family affair.

The premise of “My Mother’s Wedding” is as straightforward as its title suggests. Sisters Katharine ( Scarlett Johansson ), Victoria (Sienna Miller) and Georgina (Emily Beecham) gather for one weekend at their child

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