If the OP doesn't go, she is choosing her other children and herself over her older DD.
If the OP does go, by herself or with the other children, she is choosing this DD over her other two, and herself. She is forcing herself and her children, who are in counseling, to be around their abuser. Not having to talk to him doesn't stop the anxiety.
If the DD doesn't invite the OP's abuser, who is nothing to the DD, only he (and she) is hurt.
OP if your DD wants you and her DSis's there, you need to be unwaveringly clear and honest with her. She has invited a man who has abused both you and your other children (and from the sounds of it her as well). You've said she's probably forgotten how serious it was. It wasn't a petty argument, or a bitter break-up, this is a serious serious issue. It is not safe, emotionally at the very least, to attend with him there.
She states that she doesn't think it a big deal for her younger siblings to spend the day in the same room as him - they don't have to speak to him!
This shows just how much she doesn't understand. You don't want this to damage your relationship with her so if there's a way to show her or remind her of what he did, diaries, school reports, doctors visits, etc etc then show her. Remind her of the stress of waiting for him to explode. The anxiety, the tears. He doesn't have to do anything for it to be damaging to either you or your DD's.