Donna had 2 * 1 = <<2*1=2>>2 pieces of cake left.
Before her snack, there were 2 + 1 = <<2+1=3>>3 pieces left.
Those 3 pieces were half the leftovers, so she had 3 * 2 = <<3*2=6>>6 pieces before sharing with her brothers.
Those 6 pieces were half the party leftovers, so the cake was 2 * 6 = <<6*2=12>>12 pieces before the party.
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