@Tattycorum
Re the excusing ... Your BIL and SIL turning this all on you, the excuse of his violent childhood, your refusal to go to counseling (so you can get across what YOU think the issue is. In my head, they KNOW you won't attend, so asking you to go is just playing games and grandstanding for the counselor, and again, they get to tell their counselor that you refused because you "don't want to fix this" which makes YOU look problematic to the counselor not them), thinking your dh checking in daily will suffice to keep the kids safe, your worry about the SIL's possible breakdown, never mind that your nieces/nephews are being abused.
I get, it must be a shit place to be in, and I do not wish to add to that, but as someone whose close (female teenage) family member was being sexually abused by another close (male adult) family member, had I known, I would have shouted it to anyone who needed to hear it.
Surely counselling will help sil, if she has a good counsellor and communicates the issues to her? But she isn't communicating, is she? Because the counselor/s want you and your dh to attend. This is being seen as a relationship issues between adults, not a case of child endangerment.
If they were being honest, the counselor would have reported them to safeguarding by now.
Your dh checks, while admirable, are not going to stop any of the children being hurt in between times. It is clearly giving you a false sense that all is OK!