The question almost answers itself... bring the family together. There should be no gimmicks or trickery. I would suggest that the eldest should simply call a meeting of all family members and open the conversation. Love must be the prevailing tone for the meeting-- the person who betrayed must know that everyone in the room loves them no matter what they did to betray the family. At the same time, the family must be willing to forgive. Without love and forgiveness set from the beginning, healing will be difficult or may not happen at all. There is no need to make the betrayer feel ashamed; chances are they already feel bad enough. The family deserves answers, so ask the tough questions and hopefully the betrayer will answer with the truth. If the family is in "attack" mode, then there is very little chance that the family will learn the truth. On the other hand, if the family is in a loving and forgiving mode, then the betrayer will, hopefully, feel comfortable enough to answer.