What takes place at the family dinner in Act 1, Scene 5?

Natalie tries to get herself and Henry out of the house, but eventually joins in, along with Henry and Gabe, as Diana, moving in fast motion, serves dinner and they grab what forkfuls of food they can. At the end of the song, Diana enters with a birthday cake, lit with candles. Natalie tells Henry it's her brother's birthday and reveals that he died before she was born.