Absolutely, the relatives have no obligation to make any statements at all to the police BUT if they are subpoenaed to court they must show up and they must tell the truth and cannot take the 5th Amendment and not make a statement unless it would implicate you in a crime, so you would have to testify to the truth as there is no relative privilege in this type of case.
If you refuse to testify in court, the court can hold you in contempt and put you in jail. The police can do nothing, but the court can compel testimony. It has nothing to do at all with freedom of speech.
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