Sienna Miller was spotted relaxing with her mom, Josephine, at the Caudalie Vinotherapie Spa at the Plaza Hotel yesterday afternoon. Miller — who last July gave birth to a daughter, Marlowe, whom she had with fiancé Tom Sturridge — was overheard saying she is getting ready to walk the red carpet wearing Burberry to the Metropolitan Museum’s Costume Institute Gala on Monday. This year’s ball, celebrating the opening of the exhibition “Punk: Chaos to Couture,” is co-hosted by Anna Wintour along with Givenchy’s Riccardo Tisci.