When you said they were older teenage step children, 12 and 13 is not what I pictured.
Just as you spoke to your child about what they had done, can you not give it a little time and then sit down with the 12 and 13 year old and get DH to lead a discussion about why their complaint to SS caused such an issue in the family?
I know it’s clear to you that they are being manipulated by their mother, but at 12 and 13, they may not fully understand this. Perhaps even contact a family therapist first about how to go about this conversation, without it being accusatory, you want a conciliatory vibe. I’d suggest doing this with the counsellor, but I’m guessing the ex wife wouldn’t agree.
if the conversation goes well, then maybe start over, with some joint family outings with all children, before having the 12 and 13 year old back to your home.
while I understand wanting to cut them out for this, at 12 and 13, while being manipulated is a lot different to them being 18 and 19 and doing the same thing. They really are still children. As they come into their teen years, they’re not just navigating the usual teen hormones and rebellion, but a split family, now split even further.
While it won’t be easy, as adults, that’s where you have to be their role models for healthy relationships. I think long term though, losing contact with their half sibling and less contact with their father is a very big deal, for something where you know that these children were manipulated.