My heart goes out to your brother and his wife, for almost nothing must be harder than losing a son or daughter. God understands their grief, for He knows what it is like to lose a Son.
When tragedy strikes, people often react in one of two ways. Some are like your brother; they get angry at God and blame Him for causing it. They can't understand how God could have let it happen, and they decide He must not love them or care about them.
Others, however, turn toward God when tragedy strikes. No, they may not understand why it happened, but they know they need special strength–and they find it in God. They discover what Christians throughout the ages have discovered, that God truly is "the Father of compassion and the God of all comfort, who comforts us in all our troubles" (2 Corinthians 1:4).
Pray that your brother and his wife will sense their need of God and turn to Him. Assure them that you care, and that you understand their hurt, but then gently encourage them to turn to Christ. God loves them, and bitterness and anger only cut them off from His comfort. We may never understand some of the things that happen to us in life, but "the eternal God is your refuge, and underneath are the everlasting arms" (Deuteronomy 33:27).