You are not in a supportive marriage or family situation I am sorry to say.
I understand your feeling guilty, but sometimes showing love means allowing natural consequences to occur. What your stepson has done is so horrific that it defies the imagination. The only things worse would be murder and things of that nature.
You may have to ask yourself if you are willing to allow your stepson to dictate the safety of your biological children.
It sounds like your stepson has been set up as the "golden child" who can do no wrong. Maybe they feel badly that his biological mother is such a wreck. But to say you have "no say" is to basically state that you are not a full member of this family. That you are like a child yourself who has to abide by your husband's and in-laws' rules. That is BS and not fair to you.
To say the least!
Again I am so sorry. These children rip marriages apart. My marriage has suffered as a result of my difficult stepchildren and all of their baggage. Many more of us have similar stories to tell.
Keep posting. And call a lawyer. It doesn't mean you have to take any action yet, but it's probably time, from the way it sounds, to gather information.