I need to add that Susiestar is spot on about CPS (or DCF in my state) going after parents when one child in the household is so aggressive that they physically harm or threaten to harm another child; they don't give a rat's patootie that you are otherwise a good, conscientious parent. I know of which I speak.
Two years ago I left difficult child 2 alone in house with easy child (he was 14, she was 11) for ten minutes while I went to the store to get milk for the next day's breakfast. Ex-husband was on his way to my house to pick up difficult child 2, who resided with my ex, after difficult child 2 refused to get into the car and return to dad's (a common occurance at the time, particularly on Sunday nights).
I was steaming over difficult child 2 attempting to manipulate me and hold me hostage with his passive-aggressive dawdling, hiding in the house, running away, whenever we had to go someplace. So I made an executive decision to leave and let him stew about it until dad got there or I got back.
Well, that decision cost me. He grabbed easy child by the throat, threatened her with a pair of scissors, refused to let her use the phone or get away from him and open the door when their dad showed up (he was in the driveway waiitng for son to come out while all this was going on). I get home, see dad in the driveway, and unlock the front door to see easy child come flying into my arms sobbing hysterically about her brother...next day difficult child 2 refuses togo to school. Being proactive, I call the guidance counselor and tell him that my son is refusing to go to school and that he went to dad's office for the day (our typical arrangement if he was being oppositional and refusing to do wahtever). guidance counselor is concerned about son and gets in car and drives across town to see how son is, and finds him sleeping in the back seat of dad's pickup in the parking lot of his CPA office. It's March and cold, but son has a blanket and pillow.
The guidance counselor is horrified to see difficult child 2 sleeping in a truck when he should be at school. He hears more of the story about him threatening his younger sister and he and school SW decide to call DCF (CPS) Emergency number, and we are under investigation. They did find fault with me (one count of neglect for leaving the two children alone for one incident) but it didn't rise to any criminal charges, thank God. That was in March 2007, and DCF just closed their case in March 2009. That's right, for two straight years I had to have my parentign put under a microscope by a state agency with the power to remove my 12 yr old from my home and put her in foster care with strangers (can't go to dad's house, son is there). It was so stressful.
The social worker was a delight, but having to deal with another layer of bureaucracy, and worse, having the social worker's bosses, who carry tremendous power, basically tell exDH and I that they could not give us the tools we needed to help our child, but that we would still be judged and held accountable if our entire family did not meet some arbitrary standard of "stability" that only they are privy to, was an Orwellian nightmare I wouldn't wish on anyone.
So if nothing else but the threat of law moves your husband, let him know that if your difficult child stepson physically harms or merely threatens his sister, you can bank on another government agency getting into your business and becoming part of your life for a long time to come.