My DSis and I suffered childhood SA at the hands of our abusive F and others, OP. We repressed the memories and they came back once we had young DC. I'd had traumatic flashbacks for years which I couldn't place, which I now know were PTSD symptoms. EMDR worked well for me and I no longer have those images. The police also found the perpetrator, but the CPS decided there was insufficient evidence for a realistic chance of conviction, despite the police being convinced that they had found their man and my DSis identifying him. (This is because we didn't know him; we knew his accomplice but he had died.)
My DSis and I have both been diagnosed with PTSD. My DB is the most damaged of the three of us, yet he has no memories of the abuse. (F didn't abuse him but others did. He also joined in with the abuse, having been groomed to do so.) But the person with the most serious MH issues isn't the one who has talked about it.
We've been in a similar position to you in that our DM knew nothing about it (at times we have wondered admittedly). She's now 81, and was devastated when we told her about it a few years ago. But it didn't kill her, she coped, and your DM will very likely cope as well. It probably made it easier that my F is dead and by the time we told her she had long since got past the grief stage.
It is easier in our case, as my DSis and I were both abused so we've been able to support each other through this.
Your DSis needs your support. It must be hard to hear what she's told you about your father, but don't dismiss it because she has MH issues. Childhood SA very often leads to MH issues, so that isn't a reason not to believe her.