Hello Ashley,
abcd gave you some good advice.
I'm concerned about this behavior too.
Did you mention something about a knife?
Again, children go through phases or stages however this behavior to me doesn't seem within the acceptable range.
You do not say if the stepdaughter lives with you and her father or her mother. Sometimes the courts give the child to the mother simply because she's the mother. I know someone with a daughter who has a similar situation going on with the mother.
This is the time to nip it in the bud as the expression goes and keep reinforcing the appropriate behavior so that your step daughter changes her behavior.
I am not sure if I should be suggesting this however I will tell you that if it gets any worse with the mother's behavior if it were me I would re-consider allowing the overnight visits.
Stay strong.
Regards,
Christina