Is it possible that my dad put a mental hygiene warrant on my sister?

Probable cause?: The criteria, evidence, and basis for probable cause can vary according to locality/region. In general if a reliable and trusted person believes that another is in imminent danger of serious harm to self or others to include death a warrant could be issued for law enforcement to take that person to a doctor for evaluation. The goal is to protect the person or the public and not done haphazardly.