The answer is as follows: after criminal detention, the family members of the detained person should be informed within 24 hours after detention. Legal basis: Article 83 of the Criminal Procedure Law: When a public security organ detains a person, it must produce a detention permit. After detention, the detained person shall be immediately sent to the detention center for custody, and shall not exceed twenty-four hours. Unless it is impossible to notify or the notification of suspected crimes endangering national security or terrorist activities may hinder the investigation, the family members of the detained person shall be notified within 24 hours after detention. After the circumstances that hinder the investigation disappear, the family members of the detained person shall be notified immediately. The public security organ will inform a certain person of his criminal detention in the form of a notice of criminal detention.

According to China's criminal procedure law and other relevant laws and regulations, whether it is illegal to not inform family members in criminal detention depends on whether it meets the situation of notifying family members. The public security organ shall notify the family members within 24 hours after the arrest of a criminal, except that it is impossible to notify or the notification of a crime suspected of endangering national security or terrorist activities may hinder the investigation. Therefore, failure to inform family members is a rare exception. First of all, if you commit the above two crimes, you need to report them without hindering the investigation. Only if it interferes with the investigation can it not be notified. In addition, once the situation that hinders the investigation disappears, it needs to be notified immediately. This measure is temporary and urgent.