I can understand the fear, confusion, anger, rage and all the other feelings going on within you. In a perfect world, your family will surround and support you and help you in every way possible. Unfortunately, we don't live in that world. I confronted my abuser and was met with the usual "are you sure? Yeah, he is a bit of a jerk, but not that bad. You know, you did drugs, maybe you are confused." The list can go on. My therapist was extremely helpful and supportive. And it often really pissed me off. Like someone said, take him to court. Well, I dont know what state you are in, or what the laws are, but I was able to sue mine...my judgement was enough to pay for my therapy and almost a year of college.
But - I digress from your question. How is your relationship with your grandparents, or even one of them. What about you taking one or both of them to a trip to a park. Maybe have food, maybe just sit. But be away from the house and let them know what happened. And be ready for any reaction, the one you want or the one you don't, just be prepared. As for Christmas. You are now an adult, so you will have to make the tough choices. You can choose to go, feel uncomfortable and angry and scared, you can choose not to go, make an excuse for your absence or you can go and create tons of drama and turmoil. Personally, the last one is my least preferable. All it will do is make him look like a victim to the crazy girl with her wild accusations. It will polarize everyone and ruin the holiday's for you and everyone. Maybe mom will come around, who knows. But you need to take care of you. And the sad part is, sometimes we just need to be away from our family. Those who should support and protect us are sometimes the ones who create the most pain. What would you suggest to your daughter or your best friend. Odds are you will not get anything from him except the usual denial, and/or blame. And yes, you will see who the family supports and backs, and so often, they just want things to go back the way they were. "oh honey, it was a long time ago, let it go, get over it, etc, etc." So yes, the big dramatic accusation sounds good, it usually don't work out well. My thoughts are with you, and you have some time to work more with your therapist to gain strength and insight and get some more ideas..
hang in there...being right doesnt always feel good.