I mostly just read here and don't post, but like Cori, I would like to offer another point of view.
First looking at your stepsons's point of view. They have dad 50% of the time. Dad gets married to another woman, has another child, takes on a stepchild, and now their time with their dad, and moreso, their dad's attention is being taken away from them. I think as a child I would feel hurt if my father decided to take another child (even a bio sibling) on a special day out and I wasn't invited. This type of feeling is double in situations of divorce.
Then this other child is violent towards you or your siblings and tells lies that get you in trouble both at home and at school, and you feel blamed because you are accused of escalating it. "Ignoring" him may be more an act of self preservation and not bullying. "If I stay away from him I can't be accused of escalating him, and can't give him any excuse to hit me or blame me for something I didn't do."
Also, consider their mother's point of view. Imagine if you were sending your children to stay with their father for a period of time and they were coming home with black eyes, or requiring ER visits for head injuries, or if you were being called to the school for discipline issues that were a result of a lie told by their stepbrother.
Honestly, you should never encourage children to keep secrets from their mother. That would send up a huge red flag to me if I were their mother.
Just like you feel like you need to protect your son, she, too, probably feels the need to protect her children.
As adults, we recognize that your child has differences and behavior problems as a result of these differences, but you are expecting a level of maturity from children that isn't reasonable, especially if those children have been the victims of violent behavior and lies told to get them in trouble.
Also understand your husband's point of view. While it may be the easy solution for you to greatly reduce his time with his children, that is an awful position to put him in. He is stuck in a position where he has to choose between his wife and his children. If he ends up distancing himself from his children it will do great damage to his children, and, quite possibly, your marriage.
If his children are using therapy to complain about your son, you should listen to their complaints. I know you see this as a pile-on, but understand that your stepsons are children, too, and like your son they are hurting. Not only do they not feel safe in what should feel like their home, they feel like they are losing their father. Maybe you can go to counseling sessions with them without your biological children.
I grew up in the home where the one with the bad behavior got the lion's share of the attention and I had to tiptoe around the one with the behavior problems because it was always my fault for "escalating" things. Even when I was the victim.
I am sure that your son is in therapy and working on his behavior, and I completely understand that you feel the need to protect him, no mother ever wants to see her child hurt, but being angry at your stepchildren and their mother isn't the answer here.
Since, as you said, so much time and energy is spent on your son and your daughter, maybe you and your husband can try to carve out some time to spend just with your stepsons. Get to know them better. Develop a bond with them
Maybe your son's medication does need some fine tuning to help with the impulsive behavior and maybe you can find a family therapist that you all can agree on.
Blending families under typical circumstances is hard. You have extra obstacles, but setting up and "us and them" situation is going to make it harder.
