Can the family members go to the public security bureau to understand the case after being arrested?

You can't just do that. Generally, you can't see your family members without the approval of the public security organ after arrest. Because the arrest is still in the investigation stage, in order to ensure the smooth progress of the investigation, the relevant criminal procedure law only stipulates the right of the criminal suspect to hire a lawyer:

Article 37 of the Criminal Procedure Law stipulates that a criminal suspect may hire a lawyer to provide him with legal advice, appeal and accusation on his behalf after the first interrogation by the investigation organ or the day when compulsory measures are taken.

If the suspect is arrested, the lawyer hired can get him out on bail pending trial. In cases involving state secrets, the criminal suspect's employment of a lawyer shall be approved by the investigation organ.

Extended data:

The Criminal Procedure Law also states that:

Article 36 A defense lawyer may provide legal aid to a criminal suspect during investigation; Acting as an agent for complaints and accusations; Apply for changing compulsory measures; Ask the investigation organ about the crimes and cases suspected by the criminal suspect, and put forward opinions.

Article 38 From the date of examination and prosecution by the People's Procuratorate, defense lawyers may consult, extract and copy the case files. Other defenders may also consult, extract and copy the above materials with the permission of the people's court or the people's procuratorate.

Xishuangbanna Public Security Bureau Network-People's Republic of China (PRC) and China Criminal Law