Is it permissible to report a relative to the Committee for the Promotion of Virtue and the Prevention of Vice if he is engaged in an illicit relationship and harming his family and wife, and would this be considered a scandal?

It is not permissible to harbor ill-will towards a Muslim or to base judgments on assumptions. If you are certain that a Muslim has traveled for the sake of a girl and his relationship with her is unlawful, then you must advise him, remind him of Allah and the Last Day, and provide him with booklets and tapes related to the matter. If he does not desist, then mediate with shaykhs or friends to speak with him. If all of that is ineffective, then there is no harm in informing the Commission (Hay'ah) to prevent him from this evil by force.