Your family have been through a great deal (as will the other family affected, so my heart goes to you all, and my immediate response is to pray for you all, the other family, for healing of body, of mind, of their harmful resentments, and for you brother, your parents, your family in the continuing difficult times ahead.
I found a list of saints who were prisoners, although one significant saint isn’t mentioned as he was kidnapped, nevertheless he was prisoner, Saint Patrick. Would there be a priest who vsits the prison that holds your brother? A good Catholic chaplain? You may find advice, support, and prayer through one if available.
The gospels are a good way to begin, though if your post catches the eye of anyone in the prison ministry, they might provide you with links to appropriate and helpful books to help your brother to grown in wisdom and faith.
God bless you and your family members.