Can charges be brought against me by relatives of an elderly lady I clean for because I stuck up for her?

I privately clean for an elderly woman who recently has a 24 hour caretaker. Lot customer gets mistreated and asked to ask the caregiver questions and I did. Now the daughter in law is threatening to press charges, get me fired and saying she will make sure I never get hired anywhere. I don't work for the relatives nor do they pay me. Can they threaten me like that? I know maybe I should've minded my business but I usually can't turn away from doing the right thing. I know I'm not family but I've cleaned for this woman for years and she is like family.