قَالُوا سُبْحَانَ رَبِّنَا إِنَّا كُنَّا ظَالِمِينَ (They said, "We pronounce the purity of our Lord. No doubt, we were wrongdoers." 68:29). Neither of the brothers had paid heed to this brother's advice at that point. But when the damage was done, they admitted that Allah is Pure, Free from any qualities of weaknesses and shortcomings, and that they were the wrongdoers, in that they were unjust to the poor intending to deprive them of their share and devour it themselves.
Warning
Although the best of the three brothers offered the right advice, he joined the other brothers in following their wrong opinion. So he suffered the same loss. This indicates that anyone who prevents other people from committing sins but does not abstain from them himself and joins them in doing the wrong acts, he will suffer the same fate as the others.