Ask a Psychiatrist and Get Answers to Mental Health Questions ASAP
"Should I take my nine year old stepdaughter to a specialist?"
Yes - absolutely. During the last four years, it seems that her behavior had progressively gotten worse.
There are different reasons as to why she may be acting out ex: attention seeking, feeling inadequate, punishing the adults in her life, power struggle- seeking control.
Her behaviors are unsafe for her siblings and this acting out has to be addressed by a clinician. She may be suffering from a mood disorder or other conditions ex: Oppositional Defiant Disorder and it would be helpful to evaluate what is going on with her through a face to face meeting. That way, you'd have a better idea about what you're dealing with and she'd be offered the appropriate treatment.
You can still discipline her at home and encourage appropriate behaviors. These can be rewarded while the negative ones consequented. You've already tried for the last four years and getting some objective feedback and professional assistance is a good idea at this time, especially with the new sibling at home.