Hi there. It sounds like you are having a difficult time. If someone is making threats of violence toward you, you can report the threats to the police. Threats of violence should always be treated very seriously, regardless of who has made them.
Under federal and some state laws it is an offence to use a carriage service (such as social media sites) to menace, harass or cause offence to someone else. This means that what your partner’s daughter is doing is illegal, and carries a significant penalty. Because she is underage the situation is more complicated than if she were an adult. However the fact that she is under 18 does not mean she can harass people. This is because laws which apply to adults also apply to people under 18.
You could report the crime to the police, telling them you have received threats of violence and ongoing harassment. To assist
you, you could consider making a record of the times she has made threats of
violence and/or harassed you online. This may mean saving and printing the
messages you have received. Ongoing bullying or harassment may also be
stalking. Because of this you may also have a right to apply to a court for an
intervention order. An intervention order is an order which requires a person
to leave you alone.