OP - your DH seems like a good support, glad you have him. My DH is lovely too, but just doesn't understand. His family are all lovely and he just can't get his head around the idea of a young person being abused by a family member and the hold that still has on the victim as an adult.
DH has seen the recent letters I've got from my relation. They always start so nicely then turn nasty as the letter progresses. If it wasn't my life I'd almost be amused by the shite she says in them. I'm in my late 20s and have received such letters since the age of around 8. I thought that everyone had experiences like that.
Is interesting though as you on this thread, especially the OP have been abused by parents. In my case I am the grandchild, eldest child of her eldest child. Gran abused all her children but as they became adults and started to cut contact I became her target as I didn't know about the abuses they'd suffered and wanted a relationship with her. Big mistake!
My advice to you OP (and anyone else suffering with torn emotions re your children and your parents) is to not let your toxic parents have access to your DC, even in terms of phone calls, screen their letters if needed. My dad knew his mother was abusive, but hoped she could change and show love to her grandchildren. Turns out a leopard doesn't change its spots.
He has no idea I've been getting these letters and I don't want to tell him as a lot of the content is her trying to bad mouth my dad and twisting history. All part of her game to play us off against the other.
Any wonder I don't let her see my DC?! OP, you need to do what is best for yourself and your DC and DH. If even the police are telling you that then I think that you know what needs done.