Greg kept 1 granola bar for each of the 7 days of the week, so there were 20 - 7 = <<20-7=13>>13 bars left.
He traded 3 to his friend Pete, so there were 13 - 3 = 10 bars left.
His 2 sisters split the rest evenly, so each got 10 / 2 = <<10/2=5>>5 granola bars.
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