What is the expiation for remaining silent about a defect in a sold commodity, knowing about it, and the defect diminishing the commodity's value by 20%?

What your brother did by concealing the defect is forbidden deception. The noble Hadith indicates the prohibition of deception and concealing defects in sales. You should have advised your brother first and explained to him the impermissibility of concealing the defect. If he did not comply, then advise the buyer and explain the hidden defect to him; this is fulfilling the duty of advising. Seek forgiveness from Allah Almighty for abandoning this advice, and advise your brother to repent for what he did, and to seek out the buyer to clear himself of that right. If the buyer absolves and forgives him, then good; otherwise, he should pay him the "arsh al-ayb" (value of the defect). If it is impossible to reach the buyer, then he should give charity on his behalf, as Sheikh al-Islam Ibn Taymiyyah said: "Whoever sold a defective item, only the value of the defect from the price is unlawful for him. He must either give it to its owner or give charity on his behalf if returning it is impossible."