I know that it will probably be painful for your wife to do this, but perhaps the next time SD (stepdaughter) gets physical with her, she needs to call the cops. They'll probably put her in a juvenile detention center. Or your wife needs to ask the counselor for the phone number of a good anger management counseling service to get SD better help.

Was SD abused in any way?

Can't you and your wife consent to have her treated by a Psychiatrist?
They prescribe meds and diagnose trauma.
In our city, it takes an act of god to get a psychiatrist for a kid or teen, but in the event of an emergency - calling the police - they have to see your teen.

"Those who deny freedom to others deserve it not for themselves." ~ Abraham Lincoln

I think furie suggestion is perfect, but too late at this point.
Let them have her arrested. This girl knows what she is doing. She gets off on it. SHe needs to snap out of her bubble. If she is a true sociopath, the psychs will look at her from behind bars where she can hurt no one!
PROTECT THE TWO KIDS YOU HAVE, YOUR SS AND YOUR WIFE.
Leave her get arrested and if your wife doesn't listen, take the kids and go somewhere safe. grandma's house, but no where near sd.

Ohhh I hear you!! Loud and clear, we have been through the juvenile system, our girl has been kicked out of two group homes because they could not handle her. It's such a waste of a beautiful,intelligent and quick witted child. After yet another incident at the school, whereby the principal would have kept on having her arrested, we gave BM the choice of taking her or paying child support to the state. My husband talked of emancipating her at age 17, we have lost thousands in days off work and court costs, had our home disrupted in ways you cannot even imagine and it's just too much! Our boys do not want her back with us. As step Mom I did all I could, even though it was considered (I must add that this was first consideration on my husband's part and I do understand it) that if only SD and I could 'get along' things might be better. SD soon proved that it was not just me, it was the entire education and juvenile justice systems who could not handle her!!

What I did find very helpful was a counselor who helped me realize where the buttons were that she could find and push, I switched those off (metaphorically) and gave the girl no power over me. It's a power struggle that they try to pull people into and I fell for it!! I restored and maintained my dignity and was no longer one of her sporting events. It's so hard to deal with someone who really does not care. Sociopath is the only thing I can say! A psychiatrist examined her and he said it's emerging borderline personallity disorder. NOTHING you say or do will make a difference. My husband and I agree that we should just try to 'keep a lid on it' and she'll be moving on soon. Currently she is at her mother's, taking from her. As soon as the grass withers there she'll try and come back to us, My husband says he has had enough, this child tried to destroy our family. She outright said that was her intention!! I calmly told her that God's love for this family is way stronger than your will to destroy it girl. The last straw for my husband was all the hard work we put in to get her out of the juvenile system and she threw that back in his face by getting arrested at school, yet again, and the principal said he'd have her arrested every time she did X,Y orZ to warrant it. It's bad that parents are considered negligent when they do not collect their kids from juvenile hall, we made her wait until the last second before we collected her each time. The thing is,though, she sees juvenile hall as an occupational hazard in getting what she wants!! I really hate to sound so negative but I have to accept reality too. The doctors were very reluctant to diagnose borderline personality disorder at age 13, 14 or 15 but they finally said so at age 15. Knowing what to deal with is half the battle though, I no longer feel guilty about ignoring a child. I no longer feel guilty about the mistakes I made, this is no 'ordinary' child here!!
I hope that you get the help you need, press for a doctor to do the right thing and possibly have her committed for a while to assess her? I had to wait until the whole thing broadened out of the home before something was done. Good luck!

Sorry to break in out of the blue. Been dealing with two sd's, 14 & 17, both out of control. Mom has not provided adequate boundaries, so now they both think they can have everything their way. got frustrated with her and told her I didn't want to get caught in between her and her kids, but it's easier said than done. Everytime the girls overrun their mom, she calls for me and says she doesn't know what to do. I took her to the side and told her not to broadcast that she didn't know how to proceed,to take command, and that she needed to step up and be their mother, not their friend. She attempts to do it to a point, than backslides, and I am once again the villain. I honestly didn't know step-parenting was going to be so hard and so costly. last fall the 17 year old took off in my jeep and rolled it, than she wrecked my next vehicle. I used to work at nights, so I would be trying to sleep in the day, and she would count on this to get my keys. I feel like alot of what I built up is now wasted, and I feel trapped. I am now in college, and my Wife is assisting me by helping out monetarily, but I now don't have much of a say in anything either! :? Now what do I do?

Her BP's can have her placed in to a Residential Treatment Facility. If there is a police record, (And the police should be called EVERY single time she screws up) this will help in getting her committed. It sounds harsh I know but, I can attest to what happens when BP's don't do what NEEDS TO BE DONE. Sometimes, black and white thinking is the best way to deal with issues like these. Good luck.