Can family members pretend to be paralegals and follow lawyers to the detention center to meet? What will happen if it is discovered?

As you said, this kind of behavior is illegal. Once verified, lawyers need to bear relevant disciplinary responsibilities, resulting in collusion and other situations, and also need to bear criminal responsibility. If you just want to meet the detained person, you can apply to the procuratorial organ or the public security organ for ideological work of the criminal suspect.