Worried about my 8 year old sister

She sees a therapist already but now she has gotten worse in 21 I don’t live with my mom but she has pulled a knife on my mom twice moving it around and screaming yelling. She also has my other stepsister that 7 and my stepdad my mom is a caring mom with her and she throws tantrums and locks her self in the room my stepdad has had to break the door open lottery breaking it kicking it and hitting it that a door has a hole. She tries to hit my mom pull her clothes.