If there are any children in the family you need to tell your family. Tell her exactly what he did in a way that doesn’t sugar coat it. He wasn’t “in a dark place”- he was aroused by young girls getting raped and found it so compelling that he shared it with others. And say “I don’t know about you, but there’s no dark place I could be in where I could be aroused by children being sexually abused.” Also tell her that you will never ever be around him again, and nor will any future children of yours, and that she is being a child sexual abuse apologist. She’s literally justifying child sexual abuse because he was in a bad place. (Obviously you know all of this but I do think breaking it down to her will help her see it.)