It's hard to accept I missed this. I was supposed to protect them.
Don't do that to yourself please! Stepparents and step-siblings as SO far out of your vision and reach, they aren't even in the same state.
Plus the abusers are masterful
The step-creature's mother may have actually covered for him. There are SO SO many factors keeping you in the dark, its really not worth spending any time on shared-blame, as there IS NO SUCH THING as "shared-blame" here, now, then or whenever. CSA cases place blame on the abusers and the enablers.
Concentrate on you and the boys getting through this. You know that left to their own devices, things would not go well for them. You are doing what you need to do, and I'm very very sorry you and the boys are at this point.
I hope and pray that you find everything you and the boys need to survive and thrive.