Should a man who adopted two boys and attributed them to himself reveal the secret of their true lineage after one of them deviated in behavior, knowing that this could lead to the elder son's divorce and dire consequences for both families and the children?

Adoption that attributes a person to other than their father is impermissible. If someone did it out of ignorance, there is no sin upon them. However, if they did it with knowledge, they are sinful. Adopted individuals must be informed of their true lineage, for Allah has commanded this in His saying: "Call them by [the names of] their fathers; it is more just in the sight of Allah." It is not permissible to fear people in this matter. This is exemplified by the Prophet (peace be upon him) marrying Zaynab bint Jahsh after her divorce from Zayd ibn Harithah (may Allah be pleased with him). Persisting in adoption is legally forbidden due to the mixing of lineages, the loss of rights such as inheritance, and the entry of non-mahrams [unmarriageable kin].