Don't live with or marry the father until his daughter is older and you have a more clear-cut agreement with him about how to work out family dynamics.
get out now! this kid sounds creepy AF I've worked with kids a long time and heard some crazy s*** but this is pretty bad....pretty thought out pretty direct and like a huge sign that she intends bad for the kid who you can not actually watch every single night....get out be happy work out a custody agreement where your kid doesnt spend the night with the sibling
Have the father or Even urself set up a Dr appointment she needs to be evaluated in every aspect they can I have a bi-polar daughter even though she never acted on any of the threats she has told me she was goin kill me and all her other siblings while she was living with her father for a brief moment she said she was goin burn the house down with everyone in it sleeping and it was cause her dad treated his other kids better then her she was the oldest daughter in the house and her step mom treated her like a red head step child like for real her step mom gave my daughter her hand me down clothes that was hanging off her and it was just really ****** up sorry to say it like that but my daughter has been through hell and back and is a beautiful 15 yr old
You need to get her father on the same side as you and you need to sit her down and talk to her. If you feel you cant help ease her troubles then therapy is the next step. She is 9 and she may feel like she is left out and may need to know that she is loved the same as the other child. You are her step mother step up to the plate and begin to work with her before it does become something you cant handle. Nothing can ever get better if you dont take action. I dont know if what she is saying is enough to mark her as evil. Sometimes kids say some crazy things, and for the most part they are not meant just things they picked up on from somewhere else. I believe only you know how truly sinister she is acting. If you do feel it is that bad get her help.
But remember she is the one stuck in a hard place and she is the one that should and probably does feel pushed away from the family.