According to "People's Republic of China (PRC) Supervision Law", in the process of investigation, the supervisory organ shall listen to the statements and arguments of the respondents and respect their legitimate rights and interests.
The supervisory organ shall collect evidence according to law and shall not infringe upon the legitimate rights and interests of the person under investigation.
The supervisory organ shall protect the legitimate rights and interests of the person under investigation according to law.
Therefore, it is illegal for the Commission for Discipline Inspection to ask family members to sign without the signature of the parties concerned, and the respondent can refuse to sign.