No matter what happens to us, God still cares about us, and He is still able to give us hope and that's the most important message you can give to your brother and his wife.
This is the same message God gave to the ancient Israelites over 2,500 years ago: "'For I know the plans I have for you,' declares the Lord, 'plans to prosper you and not to harm you, plans to give you hope and a future'" (Jeremiah 29:11). They must have shaken their heads in disbelief when they first heard those words; their nation had been destroyed and they were living in slavery.
But God was at work, and He knew that someday life would change for them. And that's exactly what happened; 70 years later, the people were allowed to return to their land. Not everyone lived to see that day, but in the meantime God gave them hope, and they learned to depend on Him in faith.
I know this is a difficult time for your brother and his wife, as well as your nephew. But God still loves them, and He wants to forgive the past and put their feet on a new path His path. Sometimes God changes our situation, but sometimes He changes our hearts and helps us overcome our despair. Even if your nephew's sentence never changes, he can become the freest person in that prison if he will give his life to Christ. May you each discover God's peace as you open your hearts to Christ.