My head is spinning reading this. To all who really understand the feelings of my youngest 
I know that this situation may resonate with some that had a missing parent at their wedding, and I really do understand the depth of your feelings. Please be aware however that I would give anything to be there when she walks down the aisle, anything but the peace of mind and mental health of my babies.
There has been a lot of good advice here, thank you so much to all of you for that. I have an appointment tomorrow with both counsellors, I will be speaking to them about this situation (as they are unaware) and asking their advice. I also will ask them about the possibility of speaking with my daughter, or writing her a report type letter explaining the effects of this on her siblings.
She has witnessed the abuse, but briefly. She is aware of SS and CPS involvement, as well as the counselling, but for some reason underestimates the situation. She cares very much for her siblings and is a very bright, mature young woman, whom I am very proud of. I am just very very confused about her perception of the whole situation.
She too has had long periods of nc with my ex. He washed his hands of my grown children when we first separated, and only rekindled his relationship with them around 2 years ago. In this last period of nc my eldest child got married and he was obviously not invited. Why he decided he wanted to be involved again is anyone's guess. He seems to have firmly wormed his way back in to their affections though, and I know that they both now have a relationship with him, although I am unsure how regular the contact. They are adults and I respect their decisions. I just need to ensure that nothing negatively impacts on the little ones. They have been damaged enough.