I would go, if he said that you initiated it at the age of nine, if he said that over a decade ago, you were only young adult and the mothers were worst to believe him, you are now a young woman, if you felt strong enough with the help of friend or supportive family member, I would contact him, I would soberly look him in the face and look tall and strong with an all knowing eye and say, I hope you're not thinking of attending this birthday event, or any event in that manner, you may have pulled the wool over the parent's eyes, but i will have you arrested if you attend any event I am at, I am going to finally do what I couldn't do before, I am going to go to the police about abuse, then I am gonna get counselling, to help me get my mind round why a saddo like you abused the child that i was', you were never going to stay that child, let him know that and just then attend, see will he arrive, he has no interest in you now, you're an adult, who knows, if he's predatory, you might actually be saving another child that is in his reach, I suggest confronting him, with a witness/support, parents obviously don't or didn't understand and hoped it maybe would just go away, it never goes away until you confront the git, you are better than him, much more than you know, he's a sleaze bag, don't let him ruin enjoying your family get together