@againstmywill I am so, so sorry this has happened to you and your family. I haven't read the responses, but I wanted to respond quickly to you as you have literally described almost word for word dh and I's own experience - though (if you can believe that lightening could bloody strike twice!) for us, this happened both with MIL and her partner and my mum and her partner.
I won't go into precise detail, but basically, exactly the same. MIL's partner was convicted of CSA of his niece. MIL supported him during the trial and waited for him for 3 years while he was in prison. This is when dh was at uni, so while he knew that MIL's partner had a dodgy past and was in prison, he actually never really asked about the details (they are 'that sort' of family, sadly, the sort that only talk about the weather and what planning permission the neighbours are trying to put through). The wider family did know, but no one ever told us. Partner was on sexual harm prevention order, not allowed around children overnight unless with permission of their parents, we were banned from every staying over with our dc (they did at least stick to that, but for years I thought it was just because they didn't like me!). It finally came out 10 years later, because we were finally invited for a visit to stay with them (the SHPO was meant to be lifted - it was a 10 year one), but then the paperwork didn't get processed in time, so MIL ended up having to ring dh up and tell him that's why she was cancelling. Long story short, we went NC. We have never seen her partner again, don't go to any family events where he is present, we were NC with MIL for several years. We do see her now a few times a year, she has supervised, but restricted contact with the dc. They are never allowed to her house. I'd rather NC at all, but dh and I had to compromise. My dc know in no uncertain terms though that her partner is a bad man. What's key here is that MIL truly believes he is innocent (even though I eye roll at that because it's so unbelievable).
Anyway, fast forward 4 years from this discovery, my mum has been in a relationship with/married to her partner for 15 years. I never liked him. His own dc are NC with him (which always rings alarm bells, obviously more so in retrospect). There is just something 'off' about him that I can't put my finger on. I actually went through several years of every 6 months or so googling his name to see if I could find out something about him that might explain why I just felt something wasn't right. All those years, nothing came up. Then one day I googled again, and there it was, information about his appearance in criminal court. I couldn't see anything about the case or any other information other than his name, the court and the date. The alarm bells started clanging loudly in my head and my heart almost dropped on the floor. I knew this is why I'd been looking all these years. I hired a PI (best money I've ever spent) to find out about the case and its outcome. Long story short, he was arrested for and plead guilty to CSA against his daughter. This was a few years before he met my mum.
I had to tell my mum and I actually fully expected that I was about to destroy her life. I assumed she had no idea and dh and I actually started to make plans for if she needed to come live with us as she would have nowhere to go if she left him or he threw her out. Turns out she knew literally from their first date! She knows he did it (he told her he did), but according to her, 'everyone deserves a second chance' and she 'knew she could keep my children safe'. She was actually angry at me for finding out. Angry at me for hiring the PI. Angry at me for talking to anyone about it. I disclosed my own sexual abuse to her. She was angry at me about that too.
There was about 3 years of back and forth trying to actually get through to her about the seriousness of all of this, trying to get her to see the light (and leave him!), obviously during this time neither of them had any contact with our dc, not even cards or presents. This was just conversations between dh and I and her. Her belief at the end of the day is that he wouldn't do it again and my children were never in harm's way, and I am being ridiculous. I do know however that he has offended since - they together are members of a family naturist club for people with children, obviously, they have no children. He's groomed her to believe this is perfectly okay and normal behaviour. She doesn't see it. Her whole world revolves around him and anyone who questions anything about him becomes the enemy. Over the years, she's very vindictively cut off several very close friends, now I understand better why (friends didn't know, but they didn't like him). I tried so hard to get through to her, but the last straw was when she started spreading lies about dh and I that we had taken £100Ks from them and then run away and cut them off from seeing their grandchildren. 🙄
Anyway, we are completely NC now. I hope to never see or speak to her again, though she does try get in touch maybe a couple times a year. We have moved and she no longer knows where we live anymore, which helps.
I'm so so sorry you are going through this. It is the greatest violation I've ever experienced in my life - more traumatic that the abuse I experienced as a child, more traumatic than the multiple SA I've experienced as an adult. Having the person who should have protected you above everyone in the world violate your trust and your sense of safety for your children is the deepest cut. It shook my sense of trust in everyone and everything around me.
Sorry this is all so long! But I'm just trying to think of all the things I wish someone had said to me back when I thought I was the only person who had ever experienced this. Be compassionate with yourself. It will come in waves of disbelief and anger and sadness and blind rage. You will feel confused. You will need to figure out your new normal. Do be open and talk to people. Don't keep this a secret. These things multiply when they are kept hiding in the shadows. You will find people come from nowhere to support you and hold you up through this. I had people who I barely even knew, old work colleagues, distant family members who I usually spoke to once a year, a friend of my mum's from work 30 years ago, who scooped me up and carried me along and kept me whole. But do prepare that the people you love and who are closest to you may not do the same. Some of the people closest to me turned on dh and I. Spread malicious rumours about me. Blocked me on everything, etc. I didn't expect that either. People who I never expected to became this abuser's biggest apologists. If you have siblings or family who do feel like you and support you, keep them close and hold each other up through it all. Do when you're ready reach out and speak to a therapist. It helped so much.
I keep saying that one day I'm going to set up a charity that supports families like us who have been rocked by abuse in this way. There are charities that support children who have experienced abuse and their families. There are charities that support families of an abuser while in prison and reintegrating to the world. There is a family estrangement charity. But there isn't anything for us who have experienced our lives being torn apart by a parent who has chosen an abuser over us. There are so many of us out here. You really aren't alone. Hang in there.